iv/linux
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

docs: driver-api: Add Surface Aggregator subsystem documentation

Browse Source 
Add documentation for the Surface Aggregator subsystem and its client
drivers, giving an overview of the subsystem, its use-cases, its
internal structure and internal API, as well as its external API for
writing client drivers.

Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20201221183959.1186143-8-luzmaximilian@gmail.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
This commit is contained in:
Maximilian Luz 2020-12-21 19:39:57 +01:00 committed by Hans de Goede
parent eb0e90a820
commit 8d7792823d
10 changed files with 1529 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
Documentation/driver-api/index.rst
View File

@ -99,6 +99,7 @@ available subsections can be seen below.
rfkill
serial/index
sm501
surface_aggregator/index
switchtec
sync_file
vfio-mediated-device

38
Documentation/driver-api/surface_aggregator/client-api.rst Normal file
View File

@ -0,0 +1,38 @@
.. SPDX-License-Identifier: GPL-2.0+
===============================
Client Driver API Documentation
===============================
.. contents::
:depth: 2
Serial Hub Communication
========================
.. kernel-doc:: include/linux/surface_aggregator/serial_hub.h
.. kernel-doc:: drivers/platform/surface/aggregator/ssh_packet_layer.c
:export:
Controller and Core Interface
=============================
.. kernel-doc:: include/linux/surface_aggregator/controller.h
.. kernel-doc:: drivers/platform/surface/aggregator/controller.c
:export:
.. kernel-doc:: drivers/platform/surface/aggregator/core.c
:export:
Client Bus and Client Device API
================================
.. kernel-doc:: include/linux/surface_aggregator/device.h
.. kernel-doc:: drivers/platform/surface/aggregator/bus.c
:export:

393
Documentation/driver-api/surface_aggregator/client.rst Normal file
View File

@ -0,0 +1,393 @@
.. SPDX-License-Identifier: GPL-2.0+
.. |ssam_controller| replace:: :c:type:`struct ssam_controller <ssam_controller>`
.. |ssam_device| replace:: :c:type:`struct ssam_device <ssam_device>`
.. |ssam_device_driver| replace:: :c:type:`struct ssam_device_driver <ssam_device_driver>`
.. |ssam_client_bind| replace:: :c:func:`ssam_client_bind`
.. |ssam_client_link| replace:: :c:func:`ssam_client_link`
.. |ssam_get_controller| replace:: :c:func:`ssam_get_controller`
.. |ssam_controller_get| replace:: :c:func:`ssam_controller_get`
.. |ssam_controller_put| replace:: :c:func:`ssam_controller_put`
.. |ssam_device_alloc| replace:: :c:func:`ssam_device_alloc`
.. |ssam_device_add| replace:: :c:func:`ssam_device_add`
.. |ssam_device_remove| replace:: :c:func:`ssam_device_remove`
.. |ssam_device_driver_register| replace:: :c:func:`ssam_device_driver_register`
.. |ssam_device_driver_unregister| replace:: :c:func:`ssam_device_driver_unregister`
.. |module_ssam_device_driver| replace:: :c:func:`module_ssam_device_driver`
.. |SSAM_DEVICE| replace:: :c:func:`SSAM_DEVICE`
.. |ssam_notifier_register| replace:: :c:func:`ssam_notifier_register`
.. |ssam_notifier_unregister| replace:: :c:func:`ssam_notifier_unregister`
.. |ssam_request_sync| replace:: :c:func:`ssam_request_sync`
.. |ssam_event_mask| replace:: :c:type:`enum ssam_event_mask <ssam_event_mask>`
======================
Writing Client Drivers
======================
For the API documentation, refer to:
.. toctree::
:maxdepth: 2
client-api
Overview
========
Client drivers can be set up in two main ways, depending on how the
corresponding device is made available to the system. We specifically
differentiate between devices that are presented to the system via one of
the conventional ways, e.g. as platform devices via ACPI, and devices that
are non-discoverable and instead need to be explicitly provided by some
other mechanism, as discussed further below.
Non-SSAM Client Drivers
=======================
All communication with the SAM EC is handled via the |ssam_controller|
representing that EC to the kernel. Drivers targeting a non-SSAM device (and
thus not being a |ssam_device_driver|) need to explicitly establish a
connection/relation to that controller. This can be done via the
|ssam_client_bind| function. Said function returns a reference to the SSAM
controller, but, more importantly, also establishes a device link between
client device and controller (this can also be done separate via
|ssam_client_link|). It is important to do this, as it, first, guarantees
that the returned controller is valid for use in the client driver for as
long as this driver is bound to its device, i.e. that the driver gets
unbound before the controller ever becomes invalid, and, second, as it
ensures correct suspend/resume ordering. This setup should be done in the
driver's probe function, and may be used to defer probing in case the SSAM
subsystem is not ready yet, for example:
.. code-block:: c
static int client_driver_probe(struct platform_device *pdev)
{
struct ssam_controller *ctrl;
ctrl = ssam_client_bind(&pdev->dev);
if (IS_ERR(ctrl))
return PTR_ERR(ctrl) == -ENODEV ? -EPROBE_DEFER : PTR_ERR(ctrl);
// ...
return 0;
}
The controller may be separately obtained via |ssam_get_controller| and its
lifetime be guaranteed via |ssam_controller_get| and |ssam_controller_put|.
Note that none of these functions, however, guarantee that the controller
will not be shut down or suspended. These functions essentially only operate
on the reference, i.e. only guarantee a bare minimum of accessibility
without any guarantees at all on practical operability.
Adding SSAM Devices
===================
If a device does not already exist/is not already provided via conventional
means, it should be provided as |ssam_device| via the SSAM client device
hub. New devices can be added to this hub by entering their UID into the
corresponding registry. SSAM devices can also be manually allocated via
|ssam_device_alloc|, subsequently to which they have to be added via
|ssam_device_add| and eventually removed via |ssam_device_remove|. By
default, the parent of the device is set to the controller device provided
for allocation, however this may be changed before the device is added. Note
that, when changing the parent device, care must be taken to ensure that the
controller lifetime and suspend/resume ordering guarantees, in the default
setup provided through the parent-child relation, are preserved. If
necessary, by use of |ssam_client_link| as is done for non-SSAM client
drivers and described in more detail above.
A client device must always be removed by the party which added the
respective device before the controller shuts down. Such removal can be
guaranteed by linking the driver providing the SSAM device to the controller
via |ssam_client_link|, causing it to unbind before the controller driver
unbinds. Client devices registered with the controller as parent are
automatically removed when the controller shuts down, but this should not be
relied upon, especially as this does not extend to client devices with a
different parent.
SSAM Client Drivers
===================
SSAM client device drivers are, in essence, no different than other device
driver types. They are represented via |ssam_device_driver| and bind to a
|ssam_device| via its UID (:c:type:`struct ssam_device.uid <ssam_device>`)
member and the match table
(:c:type:`struct ssam_device_driver.match_table <ssam_device_driver>`),
which should be set when declaring the driver struct instance. Refer to the
|SSAM_DEVICE| macro documentation for more details on how to define members
of the driver's match table.
The UID for SSAM client devices consists of a ``domain``, a ``category``,
a ``target``, an ``instance``, and a ``function``. The ``domain`` is used
differentiate between physical SAM devices
(:c:type:`SSAM_DOMAIN_SERIALHUB <ssam_device_domain>`), i.e. devices that can
be accessed via the Surface Serial Hub, and virtual ones
(:c:type:`SSAM_DOMAIN_VIRTUAL <ssam_device_domain>`), such as client-device
hubs, that have no real representation on the SAM EC and are solely used on
the kernel/driver-side. For physical devices, ``category`` represents the
target category, ``target`` the target ID, and ``instance`` the instance ID
used to access the physical SAM device. In addition, ``function`` references
a specific device functionality, but has no meaning to the SAM EC. The
(default) name of a client device is generated based on its UID.
A driver instance can be registered via |ssam_device_driver_register| and
unregistered via |ssam_device_driver_unregister|. For convenience, the
|module_ssam_device_driver| macro may be used to define module init- and
exit-functions registering the driver.
The controller associated with a SSAM client device can be found in its
:c:type:`struct ssam_device.ctrl <ssam_device>` member. This reference is
guaranteed to be valid for at least as long as the client driver is bound,
but should also be valid for as long as the client device exists. Note,
however, that access outside of the bound client driver must ensure that the
controller device is not suspended while making any requests or
(un-)registering event notifiers (and thus should generally be avoided). This
is guaranteed when the controller is accessed from inside the bound client
driver.
Making Synchronous Requests
===========================
Synchronous requests are (currently) the main form of host-initiated
communication with the EC. There are a couple of ways to define and execute
such requests, however, most of them boil down to something similar as shown
in the example below. This example defines a write-read request, meaning
that the caller provides an argument to the SAM EC and receives a response.
The caller needs to know the (maximum) length of the response payload and
provide a buffer for it.
Care must be taken to ensure that any command payload data passed to the SAM
EC is provided in little-endian format and, similarly, any response payload
data received from it is converted from little-endian to host endianness.
.. code-block:: c
int perform_request(struct ssam_controller *ctrl, u32 arg, u32 *ret)
{
struct ssam_request rqst;
struct ssam_response resp;
int status;
/* Convert request argument to little-endian. */
__le32 arg_le = cpu_to_le32(arg);
__le32 ret_le = cpu_to_le32(0);
/*
* Initialize request specification. Replace this with your values.
* The rqst.payload field may be NULL if rqst.length is zero,
* indicating that the request does not have any argument.
*
* Note: The request parameters used here are not valid, i.e.
* they do not correspond to an actual SAM/EC request.
*/
rqst.target_category = SSAM_SSH_TC_SAM;
rqst.target_id = 0x01;
rqst.command_id = 0x02;
rqst.instance_id = 0x03;
rqst.flags = SSAM_REQUEST_HAS_RESPONSE;
rqst.length = sizeof(arg_le);
rqst.payload = (u8 *)&arg_le;
/* Initialize request response. */
resp.capacity = sizeof(ret_le);
resp.length = 0;
resp.pointer = (u8 *)&ret_le;
/*
* Perform actual request. The response pointer may be null in case
* the request does not have any response. This must be consistent
* with the SSAM_REQUEST_HAS_RESPONSE flag set in the specification
* above.
*/
status = ssam_request_sync(ctrl, &rqst, &resp);
/*
* Alternatively use
*
* ssam_request_sync_onstack(ctrl, &rqst, &resp, sizeof(arg_le));
*
* to perform the request, allocating the message buffer directly
* on the stack as opposed to allocation via kzalloc().
*/
/*
* Convert request response back to native format. Note that in the
* error case, this value is not touched by the SSAM core, i.e.
* 'ret_le' will be zero as specified in its initialization.
*/
*ret = le32_to_cpu(ret_le);
return status;
}
Note that |ssam_request_sync| in its essence is a wrapper over lower-level
request primitives, which may also be used to perform requests. Refer to its
implementation and documentation for more details.
An arguably more user-friendly way of defining such functions is by using
one of the generator macros, for example via:
.. code-block:: c
SSAM_DEFINE_SYNC_REQUEST_W(__ssam_tmp_perf_mode_set, __le32, {
.target_category = SSAM_SSH_TC_TMP,
.target_id = 0x01,
.command_id = 0x03,
.instance_id = 0x00,
});
This example defines a function
.. code-block:: c
int __ssam_tmp_perf_mode_set(struct ssam_controller *ctrl, const __le32 *arg);
executing the specified request, with the controller passed in when calling
said function. In this example, the argument is provided via the ``arg``
pointer. Note that the generated function allocates the message buffer on
the stack. Thus, if the argument provided via the request is large, these
kinds of macros should be avoided. Also note that, in contrast to the
previous non-macro example, this function does not do any endianness
conversion, which has to be handled by the caller. Apart from those
differences the function generated by the macro is similar to the one
provided in the non-macro example above.
The full list of such function-generating macros is
- :c:func:`SSAM_DEFINE_SYNC_REQUEST_N` for requests without return value and
without argument.
- :c:func:`SSAM_DEFINE_SYNC_REQUEST_R` for requests with return value but no
argument.
- :c:func:`SSAM_DEFINE_SYNC_REQUEST_W` for requests without return value but
with argument.
Refer to their respective documentation for more details. For each one of
these macros, a special variant is provided, which targets request types
applicable to multiple instances of the same device type:
- :c:func:`SSAM_DEFINE_SYNC_REQUEST_MD_N`
- :c:func:`SSAM_DEFINE_SYNC_REQUEST_MD_R`
- :c:func:`SSAM_DEFINE_SYNC_REQUEST_MD_W`
The difference of those macros to the previously mentioned versions is, that
the device target and instance IDs are not fixed for the generated function,
but instead have to be provided by the caller of said function.
Additionally, variants for direct use with client devices, i.e.
|ssam_device|, are also provided. These can, for example, be used as
follows:
.. code-block:: c
SSAM_DEFINE_SYNC_REQUEST_CL_R(ssam_bat_get_sta, __le32, {
.target_category = SSAM_SSH_TC_BAT,
.command_id = 0x01,
});
This invocation of the macro defines a function
.. code-block:: c
int ssam_bat_get_sta(struct ssam_device *sdev, __le32 *ret);
executing the specified request, using the device IDs and controller given
in the client device. The full list of such macros for client devices is:
- :c:func:`SSAM_DEFINE_SYNC_REQUEST_CL_N`
- :c:func:`SSAM_DEFINE_SYNC_REQUEST_CL_R`
- :c:func:`SSAM_DEFINE_SYNC_REQUEST_CL_W`
Handling Events
===============
To receive events from the SAM EC, an event notifier must be registered for
the desired event via |ssam_notifier_register|. The notifier must be
unregistered via |ssam_notifier_unregister| once it is not required any
more.
Event notifiers are registered by providing (at minimum) a callback to call
in case an event has been received, the registry specifying how the event
should be enabled, an event ID specifying for which target category and,
optionally and depending on the registry used, for which instance ID events
should be enabled, and finally, flags describing how the EC will send these
events. If the specific registry does not enable events by instance ID, the
instance ID must be set to zero. Additionally, a priority for the respective
notifier may be specified, which determines its order in relation to any
other notifier registered for the same target category.
By default, event notifiers will receive all events for the specific target
category, regardless of the instance ID specified when registering the
notifier. The core may be instructed to only call a notifier if the target
ID or instance ID (or both) of the event match the ones implied by the
notifier IDs (in case of target ID, the target ID of the registry), by
providing an event mask (see |ssam_event_mask|).
In general, the target ID of the registry is also the target ID of the
enabled event (with the notable exception being keyboard input events on the
Surface Laptop 1 and 2, which are enabled via a registry with target ID 1,
but provide events with target ID 2).
A full example for registering an event notifier and handling received
events is provided below:
.. code-block:: c
u32 notifier_callback(struct ssam_event_notifier *nf,
const struct ssam_event *event)
{
int status = ...
/* Handle the event here ... */
/* Convert return value and indicate that we handled the event. */
return ssam_notifier_from_errno(status) | SSAM_NOTIF_HANDLED;
}
int setup_notifier(struct ssam_device *sdev,
struct ssam_event_notifier *nf)
{
/* Set priority wrt. other handlers of same target category. */
nf->base.priority = 1;
/* Set event/notifier callback. */
nf->base.fn = notifier_callback;
/* Specify event registry, i.e. how events get enabled/disabled. */
nf->event.reg = SSAM_EVENT_REGISTRY_KIP;
/* Specify which event to enable/disable */
nf->event.id.target_category = sdev->uid.category;
nf->event.id.instance = sdev->uid.instance;
/*
* Specify for which events the notifier callback gets executed.
* This essentially tells the core if it can skip notifiers that
* don't have target or instance IDs matching those of the event.
*/
nf->event.mask = SSAM_EVENT_MASK_STRICT;
/* Specify event flags. */
nf->event.flags = SSAM_EVENT_SEQUENCED;
return ssam_notifier_register(sdev->ctrl, nf);
}
Multiple event notifiers can be registered for the same event. The event
handler core takes care of enabling and disabling events when notifiers are
registered and unregistered, by keeping track of how many notifiers for a
specific event (combination of registry, event target category, and event
instance ID) are currently registered. This means that a specific event will
be enabled when the first notifier for it is being registered and disabled
when the last notifier for it is being unregistered. Note that the event
flags are therefore only used on the first registered notifier, however, one
should take care that notifiers for a specific event are always registered
with the same flag and it is considered a bug to do otherwise.

10
Documentation/driver-api/surface_aggregator/clients/index.rst Normal file
View File

@ -0,0 +1,10 @@
.. SPDX-License-Identifier: GPL-2.0+
===========================
Client Driver Documentation
===========================
This is the documentation for client drivers themselves. Refer to
:doc:`../client` for documentation on how to write client drivers.
.. Place documentation for individual client drivers here.

21
Documentation/driver-api/surface_aggregator/index.rst Normal file
View File

@ -0,0 +1,21 @@
.. SPDX-License-Identifier: GPL-2.0+
=======================================
Surface System Aggregator Module (SSAM)
=======================================
.. toctree::
:maxdepth: 2
overview
client
clients/index
ssh
internal
.. only:: subproject and html
Indices
=======
* :ref:`genindex`

67
Documentation/driver-api/surface_aggregator/internal-api.rst Normal file
View File

@ -0,0 +1,67 @@
.. SPDX-License-Identifier: GPL-2.0+
==========================
Internal API Documentation
==========================
.. contents::
:depth: 2
Packet Transport Layer
======================
.. kernel-doc:: drivers/platform/surface/aggregator/ssh_parser.h
:internal:
.. kernel-doc:: drivers/platform/surface/aggregator/ssh_parser.c
:internal:
.. kernel-doc:: drivers/platform/surface/aggregator/ssh_msgb.h
:internal:
.. kernel-doc:: drivers/platform/surface/aggregator/ssh_packet_layer.h
:internal:
.. kernel-doc:: drivers/platform/surface/aggregator/ssh_packet_layer.c
:internal:
Request Transport Layer
=======================
.. kernel-doc:: drivers/platform/surface/aggregator/ssh_request_layer.h
:internal:
.. kernel-doc:: drivers/platform/surface/aggregator/ssh_request_layer.c
:internal:
Controller
==========
.. kernel-doc:: drivers/platform/surface/aggregator/controller.h
:internal:
.. kernel-doc:: drivers/platform/surface/aggregator/controller.c
:internal:
Client Device Bus
=================
.. kernel-doc:: drivers/platform/surface/aggregator/bus.c
:internal:
Core
====
.. kernel-doc:: drivers/platform/surface/aggregator/core.c
:internal:
Trace Helpers
=============
.. kernel-doc:: drivers/platform/surface/aggregator/trace.h

577
Documentation/driver-api/surface_aggregator/internal.rst Normal file
View File

@ -0,0 +1,577 @@
.. SPDX-License-Identifier: GPL-2.0+
.. |ssh_ptl| replace:: :c:type:`struct ssh_ptl <ssh_ptl>`
.. |ssh_ptl_submit| replace:: :c:func:`ssh_ptl_submit`
.. |ssh_ptl_cancel| replace:: :c:func:`ssh_ptl_cancel`
.. |ssh_ptl_shutdown| replace:: :c:func:`ssh_ptl_shutdown`
.. |ssh_ptl_rx_rcvbuf| replace:: :c:func:`ssh_ptl_rx_rcvbuf`
.. |ssh_rtl| replace:: :c:type:`struct ssh_rtl <ssh_rtl>`
.. |ssh_rtl_submit| replace:: :c:func:`ssh_rtl_submit`
.. |ssh_rtl_cancel| replace:: :c:func:`ssh_rtl_cancel`
.. |ssh_rtl_shutdown| replace:: :c:func:`ssh_rtl_shutdown`
.. |ssh_packet| replace:: :c:type:`struct ssh_packet <ssh_packet>`
.. |ssh_packet_get| replace:: :c:func:`ssh_packet_get`
.. |ssh_packet_put| replace:: :c:func:`ssh_packet_put`
.. |ssh_packet_ops| replace:: :c:type:`struct ssh_packet_ops <ssh_packet_ops>`
.. |ssh_packet_base_priority| replace:: :c:type:`enum ssh_packet_base_priority <ssh_packet_base_priority>`
.. |ssh_packet_flags| replace:: :c:type:`enum ssh_packet_flags <ssh_packet_flags>`
.. |SSH_PACKET_PRIORITY| replace:: :c:func:`SSH_PACKET_PRIORITY`
.. |ssh_frame| replace:: :c:type:`struct ssh_frame <ssh_frame>`
.. |ssh_command| replace:: :c:type:`struct ssh_command <ssh_command>`
.. |ssh_request| replace:: :c:type:`struct ssh_request <ssh_request>`
.. |ssh_request_get| replace:: :c:func:`ssh_request_get`
.. |ssh_request_put| replace:: :c:func:`ssh_request_put`
.. |ssh_request_ops| replace:: :c:type:`struct ssh_request_ops <ssh_request_ops>`
.. |ssh_request_init| replace:: :c:func:`ssh_request_init`
.. |ssh_request_flags| replace:: :c:type:`enum ssh_request_flags <ssh_request_flags>`
.. |ssam_controller| replace:: :c:type:`struct ssam_controller <ssam_controller>`
.. |ssam_device| replace:: :c:type:`struct ssam_device <ssam_device>`
.. |ssam_device_driver| replace:: :c:type:`struct ssam_device_driver <ssam_device_driver>`
.. |ssam_client_bind| replace:: :c:func:`ssam_client_bind`
.. |ssam_client_link| replace:: :c:func:`ssam_client_link`
.. |ssam_request_sync| replace:: :c:type:`struct ssam_request_sync <ssam_request_sync>`
.. |ssam_event_registry| replace:: :c:type:`struct ssam_event_registry <ssam_event_registry>`
.. |ssam_event_id| replace:: :c:type:`struct ssam_event_id <ssam_event_id>`
.. |ssam_nf| replace:: :c:type:`struct ssam_nf <ssam_nf>`
.. |ssam_nf_refcount_inc| replace:: :c:func:`ssam_nf_refcount_inc`
.. |ssam_nf_refcount_dec| replace:: :c:func:`ssam_nf_refcount_dec`
.. |ssam_notifier_register| replace:: :c:func:`ssam_notifier_register`
.. |ssam_notifier_unregister| replace:: :c:func:`ssam_notifier_unregister`
.. |ssam_cplt| replace:: :c:type:`struct ssam_cplt <ssam_cplt>`
.. |ssam_event_queue| replace:: :c:type:`struct ssam_event_queue <ssam_event_queue>`
.. |ssam_request_sync_submit| replace:: :c:func:`ssam_request_sync_submit`
=====================
Core Driver Internals
=====================
Architectural overview of the Surface System Aggregator Module (SSAM) core
and Surface Serial Hub (SSH) driver. For the API documentation, refer to:
.. toctree::
:maxdepth: 2
internal-api
Overview
========
The SSAM core implementation is structured in layers, somewhat following the
SSH protocol structure:
Lower-level packet transport is implemented in the *packet transport layer
(PTL)*, directly building on top of the serial device (serdev)
infrastructure of the kernel. As the name indicates, this layer deals with
the packet transport logic and handles things like packet validation, packet
acknowledgment (ACKing), packet (retransmission) timeouts, and relaying
packet payloads to higher-level layers.
Above this sits the *request transport layer (RTL)*. This layer is centered
around command-type packet payloads, i.e. requests (sent from host to EC),
responses of the EC to those requests, and events (sent from EC to host).
It, specifically, distinguishes events from request responses, matches
responses to their corresponding requests, and implements request timeouts.
The *controller* layer is building on top of this and essentially decides
how request responses and, especially, events are dealt with. It provides an
event notifier system, handles event activation/deactivation, provides a
workqueue for event and asynchronous request completion, and also manages
the message counters required for building command messages (``SEQ``,
``RQID``). This layer basically provides a fundamental interface to the SAM
EC for use in other kernel drivers.
While the controller layer already provides an interface for other kernel
drivers, the client *bus* extends this interface to provide support for
native SSAM devices, i.e. devices that are not defined in ACPI and not
implemented as platform devices, via |ssam_device| and |ssam_device_driver|
simplify management of client devices and client drivers.
Refer to :doc:`client` for documentation regarding the client device/driver
API and interface options for other kernel drivers. It is recommended to
familiarize oneself with that chapter and the :doc:`ssh` before continuing
with the architectural overview below.
Packet Transport Layer
======================
The packet transport layer is represented via |ssh_ptl| and is structured
around the following key concepts:
Packets
-------
Packets are the fundamental transmission unit of the SSH protocol. They are
managed by the packet transport layer, which is essentially the lowest layer
of the driver and is built upon by other components of the SSAM core.
Packets to be transmitted by the SSAM core are represented via |ssh_packet|
(in contrast, packets received by the core do not have any specific
structure and are managed entirely via the raw |ssh_frame|).
This structure contains the required fields to manage the packet inside the
transport layer, as well as a reference to the buffer containing the data to
be transmitted (i.e. the message wrapped in |ssh_frame|). Most notably, it
contains an internal reference count, which is used for managing its
lifetime (accessible via |ssh_packet_get| and |ssh_packet_put|). When this
counter reaches zero, the ``release()`` callback provided to the packet via
its |ssh_packet_ops| reference is executed, which may then deallocate the
packet or its enclosing structure (e.g. |ssh_request|).
In addition to the ``release`` callback, the |ssh_packet_ops| reference also
provides a ``complete()`` callback, which is run once the packet has been
completed and provides the status of this completion, i.e. zero on success
or a negative errno value in case of an error. Once the packet has been
submitted to the packet transport layer, the ``complete()`` callback is
always guaranteed to be executed before the ``release()`` callback, i.e. the
packet will always be completed, either successfully, with an error, or due
to cancellation, before it will be released.
The state of a packet is managed via its ``state`` flags
(|ssh_packet_flags|), which also contains the packet type. In particular,
the following bits are noteworthy:
* ``SSH_PACKET_SF_LOCKED_BIT``: This bit is set when completion, either
through error or success, is imminent. It indicates that no further
references of the packet should be taken and any existing references
should be dropped as soon as possible. The process setting this bit is
responsible for removing any references to this packet from the packet
queue and pending set.
* ``SSH_PACKET_SF_COMPLETED_BIT``: This bit is set by the process running the
``complete()`` callback and is used to ensure that this callback only runs
once.
* ``SSH_PACKET_SF_QUEUED_BIT``: This bit is set when the packet is queued on
the packet queue and cleared when it is dequeued.
* ``SSH_PACKET_SF_PENDING_BIT``: This bit is set when the packet is added to
the pending set and cleared when it is removed from it.
Packet Queue
------------
The packet queue is the first of the two fundamental collections in the
packet transport layer. It is a priority queue, with priority of the
respective packets based on the packet type (major) and number of tries
(minor). See |SSH_PACKET_PRIORITY| for more details on the priority value.
All packets to be transmitted by the transport layer must be submitted to
this queue via |ssh_ptl_submit|. Note that this includes control packets
sent by the transport layer itself. Internally, data packets can be
re-submitted to this queue due to timeouts or NAK packets sent by the EC.
Pending Set
-----------
The pending set is the second of the two fundamental collections in the
packet transport layer. It stores references to packets that have already
been transmitted, but wait for acknowledgment (e.g. the corresponding ACK
packet) by the EC.
Note that a packet may both be pending and queued if it has been
re-submitted due to a packet acknowledgment timeout or NAK. On such a
re-submission, packets are not removed from the pending set.
Transmitter Thread
------------------
The transmitter thread is responsible for most of the actual work regarding
packet transmission. In each iteration, it (waits for and) checks if the
next packet on the queue (if any) can be transmitted and, if so, removes it
from the queue and increments its counter for the number of transmission
attempts, i.e. tries. If the packet is sequenced, i.e. requires an ACK by
the EC, the packet is added to the pending set. Next, the packet's data is
submitted to the serdev subsystem. In case of an error or timeout during
this submission, the packet is completed by the transmitter thread with the
status value of the callback set accordingly. In case the packet is
unsequenced, i.e. does not require an ACK by the EC, the packet is completed
with success on the transmitter thread.
Transmission of sequenced packets is limited by the number of concurrently
pending packets, i.e. a limit on how many packets may be waiting for an ACK
from the EC in parallel. This limit is currently set to one (see :doc:`ssh`
for the reasoning behind this). Control packets (i.e. ACK and NAK) can
always be transmitted.
Receiver Thread
---------------
Any data received from the EC is put into a FIFO buffer for further
processing. This processing happens on the receiver thread. The receiver
thread parses and validates the received message into its |ssh_frame| and
corresponding payload. It prepares and submits the necessary ACK (and on
validation error or invalid data NAK) packets for the received messages.
This thread also handles further processing, such as matching ACK messages
to the corresponding pending packet (via sequence ID) and completing it, as
well as initiating re-submission of all currently pending packets on
receival of a NAK message (re-submission in case of a NAK is similar to
re-submission due to timeout, see below for more details on that). Note that
the successful completion of a sequenced packet will always run on the
receiver thread (whereas any failure-indicating completion will run on the
process where the failure occurred).
Any payload data is forwarded via a callback to the next upper layer, i.e.
the request transport layer.
Timeout Reaper
--------------
The packet acknowledgment timeout is a per-packet timeout for sequenced
packets, started when the respective packet begins (re-)transmission (i.e.
this timeout is armed once per transmission attempt on the transmitter
thread). It is used to trigger re-submission or, when the number of tries
has been exceeded, cancellation of the packet in question.
This timeout is handled via a dedicated reaper task, which is essentially a
work item (re-)scheduled to run when the next packet is set to time out. The
work item then checks the set of pending packets for any packets that have
exceeded the timeout and, if there are any remaining packets, re-schedules
itself to the next appropriate point in time.
If a timeout has been detected by the reaper, the packet will either be
re-submitted if it still has some remaining tries left, or completed with
``-ETIMEDOUT`` as status if not. Note that re-submission, in this case and
triggered by receival of a NAK, means that the packet is added to the queue
with a now incremented number of tries, yielding a higher priority. The
timeout for the packet will be disabled until the next transmission attempt
and the packet remains on the pending set.
Note that due to transmission and packet acknowledgment timeouts, the packet
transport layer is always guaranteed to make progress, if only through
timing out packets, and will never fully block.
Concurrency and Locking
-----------------------
There are two main locks in the packet transport layer: One guarding access
to the packet queue and one guarding access to the pending set. These
collections may only be accessed and modified under the respective lock. If
access to both collections is needed, the pending lock must be acquired
before the queue lock to avoid deadlocks.
In addition to guarding the collections, after initial packet submission
certain packet fields may only be accessed under one of the locks.
Specifically, the packet priority must only be accessed while holding the
queue lock and the packet timestamp must only be accessed while holding the
pending lock.
Other parts of the packet transport layer are guarded independently. State
flags are managed by atomic bit operations and, if necessary, memory
barriers. Modifications to the timeout reaper work item and expiration date
are guarded by their own lock.
The reference of the packet to the packet transport layer (``ptl``) is
somewhat special. It is either set when the upper layer request is submitted
or, if there is none, when the packet is first submitted. After it is set,
it will not change its value. Functions that may run concurrently with
submission, i.e. cancellation, can not rely on the ``ptl`` reference to be
set. Access to it in these functions is guarded by ``READ_ONCE()``, whereas
setting ``ptl`` is equally guarded with ``WRITE_ONCE()`` for symmetry.
Some packet fields may be read outside of the respective locks guarding
them, specifically priority and state for tracing. In those cases, proper
access is ensured by employing ``WRITE_ONCE()`` and ``READ_ONCE()``. Such
read-only access is only allowed when stale values are not critical.
With respect to the interface for higher layers, packet submission
(|ssh_ptl_submit|), packet cancellation (|ssh_ptl_cancel|), data receival
(|ssh_ptl_rx_rcvbuf|), and layer shutdown (|ssh_ptl_shutdown|) may always be
executed concurrently with respect to each other. Note that packet
submission may not run concurrently with itself for the same packet.
Equally, shutdown and data receival may also not run concurrently with
themselves (but may run concurrently with each other).
Request Transport Layer
=======================
The request transport layer is represented via |ssh_rtl| and builds on top
of the packet transport layer. It deals with requests, i.e. SSH packets sent
by the host containing a |ssh_command| as frame payload. This layer
separates responses to requests from events, which are also sent by the EC
via a |ssh_command| payload. While responses are handled in this layer,
events are relayed to the next upper layer, i.e. the controller layer, via
the corresponding callback. The request transport layer is structured around
the following key concepts:
Request
-------
Requests are packets with a command-type payload, sent from host to EC to
query data from or trigger an action on it (or both simultaneously). They
are represented by |ssh_request|, wrapping the underlying |ssh_packet|
storing its message data (i.e. SSH frame with command payload). Note that
all top-level representations, e.g. |ssam_request_sync| are built upon this
struct.
As |ssh_request| extends |ssh_packet|, its lifetime is also managed by the
reference counter inside the packet struct (which can be accessed via
|ssh_request_get| and |ssh_request_put|). Once the counter reaches zero, the
``release()`` callback of the |ssh_request_ops| reference of the request is
called.
Requests can have an optional response that is equally sent via a SSH
message with command-type payload (from EC to host). The party constructing
the request must know if a response is expected and mark this in the request
flags provided to |ssh_request_init|, so that the request transport layer
can wait for this response.
Similar to |ssh_packet|, |ssh_request| also has a ``complete()`` callback
provided via its request ops reference and is guaranteed to be completed
before it is released once it has been submitted to the request transport
layer via |ssh_rtl_submit|. For a request without a response, successful
completion will occur once the underlying packet has been successfully
transmitted by the packet transport layer (i.e. from within the packet
completion callback). For a request with response, successful completion
will occur once the response has been received and matched to the request
via its request ID (which happens on the packet layer's data-received
callback running on the receiver thread). If the request is completed with
an error, the status value will be set to the corresponding (negative) errno
value.
The state of a request is again managed via its ``state`` flags
(|ssh_request_flags|), which also encode the request type. In particular,
the following bits are noteworthy:
* ``SSH_REQUEST_SF_LOCKED_BIT``: This bit is set when completion, either
through error or success, is imminent. It indicates that no further
references of the request should be taken and any existing references
should be dropped as soon as possible. The process setting this bit is
responsible for removing any references to this request from the request
queue and pending set.
* ``SSH_REQUEST_SF_COMPLETED_BIT``: This bit is set by the process running the
``complete()`` callback and is used to ensure that this callback only runs
once.
* ``SSH_REQUEST_SF_QUEUED_BIT``: This bit is set when the request is queued on
the request queue and cleared when it is dequeued.
* ``SSH_REQUEST_SF_PENDING_BIT``: This bit is set when the request is added to
the pending set and cleared when it is removed from it.
Request Queue
-------------
The request queue is the first of the two fundamental collections in the
request transport layer. In contrast to the packet queue of the packet
transport layer, it is not a priority queue and the simple first come first
serve principle applies.
All requests to be transmitted by the request transport layer must be
submitted to this queue via |ssh_rtl_submit|. Once submitted, requests may
not be re-submitted, and will not be re-submitted automatically on timeout.
Instead, the request is completed with a timeout error. If desired, the
caller can create and submit a new request for another try, but it must not
submit the same request again.
Pending Set
-----------
The pending set is the second of the two fundamental collections in the
request transport layer. This collection stores references to all pending
requests, i.e. requests awaiting a response from the EC (similar to what the
pending set of the packet transport layer does for packets).
Transmitter Task
----------------
The transmitter task is scheduled when a new request is available for
transmission. It checks if the next request on the request queue can be
transmitted and, if so, submits its underlying packet to the packet
transport layer. This check ensures that only a limited number of
requests can be pending, i.e. waiting for a response, at the same time. If
the request requires a response, the request is added to the pending set
before its packet is submitted.
Packet Completion Callback
--------------------------
The packet completion callback is executed once the underlying packet of a
request has been completed. In case of an error completion, the
corresponding request is completed with the error value provided in this
callback.
On successful packet completion, further processing depends on the request.
If the request expects a response, it is marked as transmitted and the
request timeout is started. If the request does not expect a response, it is
completed with success.
Data-Received Callback
----------------------
The data received callback notifies the request transport layer of data
being received by the underlying packet transport layer via a data-type
frame. In general, this is expected to be a command-type payload.
If the request ID of the command is one of the request IDs reserved for
events (one to ``SSH_NUM_EVENTS``, inclusively), it is forwarded to the
event callback registered in the request transport layer. If the request ID
indicates a response to a request, the respective request is looked up in
the pending set and, if found and marked as transmitted, completed with
success.
Timeout Reaper
--------------
The request-response-timeout is a per-request timeout for requests expecting
a response. It is used to ensure that a request does not wait indefinitely
on a response from the EC and is started after the underlying packet has
been successfully completed.
This timeout is, similar to the packet acknowledgment timeout on the packet
transport layer, handled via a dedicated reaper task. This task is
essentially a work-item (re-)scheduled to run when the next request is set
to time out. The work item then scans the set of pending requests for any
requests that have timed out and completes them with ``-ETIMEDOUT`` as
status. Requests will not be re-submitted automatically. Instead, the issuer
of the request must construct and submit a new request, if so desired.
Note that this timeout, in combination with packet transmission and
acknowledgment timeouts, guarantees that the request layer will always make
progress, even if only through timing out packets, and never fully block.
Concurrency and Locking
-----------------------
Similar to the packet transport layer, there are two main locks in the
request transport layer: One guarding access to the request queue and one
guarding access to the pending set. These collections may only be accessed
and modified under the respective lock.
Other parts of the request transport layer are guarded independently. State
flags are (again) managed by atomic bit operations and, if necessary, memory
barriers. Modifications to the timeout reaper work item and expiration date
are guarded by their own lock.
Some request fields may be read outside of the respective locks guarding
them, specifically the state for tracing. In those cases, proper access is
ensured by employing ``WRITE_ONCE()`` and ``READ_ONCE()``. Such read-only
access is only allowed when stale values are not critical.
With respect to the interface for higher layers, request submission
(|ssh_rtl_submit|), request cancellation (|ssh_rtl_cancel|), and layer
shutdown (|ssh_rtl_shutdown|) may always be executed concurrently with
respect to each other. Note that request submission may not run concurrently
with itself for the same request (and also may only be called once per
request). Equally, shutdown may also not run concurrently with itself.
Controller Layer
================
The controller layer extends on the request transport layer to provide an
easy-to-use interface for client drivers. It is represented by
|ssam_controller| and the SSH driver. While the lower level transport layers
take care of transmitting and handling packets and requests, the controller
layer takes on more of a management role. Specifically, it handles device
initialization, power management, and event handling, including event
delivery and registration via the (event) completion system (|ssam_cplt|).
Event Registration
------------------
In general, an event (or rather a class of events) has to be explicitly
requested by the host before the EC will send it (HID input events seem to
be the exception). This is done via an event-enable request (similarly,
events should be disabled via an event-disable request once no longer
desired).
The specific request used to enable (or disable) an event is given via an
event registry, i.e. the governing authority of this event (so to speak),
represented by |ssam_event_registry|. As parameters to this request, the
target category and, depending on the event registry, instance ID of the
event to be enabled must be provided. This (optional) instance ID must be
zero if the registry does not use it. Together, target category and instance
ID form the event ID, represented by |ssam_event_id|. In short, both, event
registry and event ID, are required to uniquely identify a respective class
of events.
Note that a further *request ID* parameter must be provided for the
enable-event request. This parameter does not influence the class of events
being enabled, but instead is set as the request ID (RQID) on each event of
this class sent by the EC. It is used to identify events (as a limited
number of request IDs is reserved for use in events only, specifically one
to ``SSH_NUM_EVENTS`` inclusively) and also map events to their specific
class. Currently, the controller always sets this parameter to the target
category specified in |ssam_event_id|.
As multiple client drivers may rely on the same (or overlapping) classes of
events and enable/disable calls are strictly binary (i.e. on/off), the
controller has to manage access to these events. It does so via reference
counting, storing the counter inside an RB-tree based mapping with event
registry and ID as key (there is no known list of valid event registry and
event ID combinations). See |ssam_nf|, |ssam_nf_refcount_inc|, and
|ssam_nf_refcount_dec| for details.
This management is done together with notifier registration (described in
the next section) via the top-level |ssam_notifier_register| and
|ssam_notifier_unregister| functions.
Event Delivery
--------------
To receive events, a client driver has to register an event notifier via
|ssam_notifier_register|. This increments the reference counter for that
specific class of events (as detailed in the previous section), enables the
class on the EC (if it has not been enabled already), and installs the
provided notifier callback.
Notifier callbacks are stored in lists, with one (RCU) list per target
category (provided via the event ID; NB: there is a fixed known number of
target categories). There is no known association from the combination of
event registry and event ID to the command data (target ID, target category,
command ID, and instance ID) that can be provided by an event class, apart
from target category and instance ID given via the event ID.
Note that due to the way notifiers are (or rather have to be) stored, client
drivers may receive events that they have not requested and need to account
for them. Specifically, they will, by default, receive all events from the
same target category. To simplify dealing with this, filtering of events by
target ID (provided via the event registry) and instance ID (provided via
the event ID) can be requested when registering a notifier. This filtering
is applied when iterating over the notifiers at the time they are executed.
All notifier callbacks are executed on a dedicated workqueue, the so-called
completion workqueue. After an event has been received via the callback
installed in the request layer (running on the receiver thread of the packet
transport layer), it will be put on its respective event queue
(|ssam_event_queue|). From this event queue the completion work item of that
queue (running on the completion workqueue) will pick up the event and
execute the notifier callback. This is done to avoid blocking on the
receiver thread.
There is one event queue per combination of target ID and target category.
This is done to ensure that notifier callbacks are executed in sequence for
events of the same target ID and target category. Callbacks can be executed
in parallel for events with a different combination of target ID and target
category.
Concurrency and Locking
-----------------------
Most of the concurrency related safety guarantees of the controller are
provided by the lower-level request transport layer. In addition to this,
event (un-)registration is guarded by its own lock.
Access to the controller state is guarded by the state lock. This lock is a
read/write semaphore. The reader part can be used to ensure that the state
does not change while functions depending on the state to stay the same
(e.g. |ssam_notifier_register|, |ssam_notifier_unregister|,
|ssam_request_sync_submit|, and derivatives) are executed and this guarantee
is not already provided otherwise (e.g. through |ssam_client_bind| or
|ssam_client_link|). The writer part guards any transitions that will change
the state, i.e. initialization, destruction, suspension, and resumption.
The controller state may be accessed (read-only) outside the state lock for
smoke-testing against invalid API usage (e.g. in |ssam_request_sync_submit|).
Note that such checks are not supposed to (and will not) protect against all
invalid usages, but rather aim to help catch them. In those cases, proper
variable access is ensured by employing ``WRITE_ONCE()`` and ``READ_ONCE()``.
Assuming any preconditions on the state not changing have been satisfied,
all non-initialization and non-shutdown functions may run concurrently with
each other. This includes |ssam_notifier_register|, |ssam_notifier_unregister|,
|ssam_request_sync_submit|, as well as all functions building on top of those.

77
Documentation/driver-api/surface_aggregator/overview.rst Normal file
View File

@ -0,0 +1,77 @@
.. SPDX-License-Identifier: GPL-2.0+
========
Overview
========
The Surface/System Aggregator Module (SAM, SSAM) is an (arguably *the*)
embedded controller (EC) on Microsoft Surface devices. It has been originally
introduced on 4th generation devices (Surface Pro 4, Surface Book 1), but
its responsibilities and feature-set have since been expanded significantly
with the following generations.
Features and Integration
========================
Not much is currently known about SAM on 4th generation devices (Surface Pro
4, Surface Book 1), due to the use of a different communication interface
between host and EC (as detailed below). On 5th (Surface Pro 2017, Surface
Book 2, Surface Laptop 1) and later generation devices, SAM is responsible
for providing battery information (both current status and static values,
such as maximum capacity etc.), as well as an assortment of temperature
sensors (e.g. skin temperature) and cooling/performance-mode setting to the
host. On the Surface Book 2, specifically, it additionally provides an
interface for properly handling clipboard detachment (i.e. separating the
display part from the keyboard part of the device), on the Surface Laptop 1
and 2 it is required for keyboard HID input. This HID subsystem has been
restructured for 7th generation devices and on those, specifically Surface
Laptop 3 and Surface Book 3, is responsible for all major HID input (i.e.
keyboard and touchpad).
While features have not changed much on a coarse level since the 5th
generation, internal interfaces have undergone some rather large changes. On
5th and 6th generation devices, both battery and temperature information is
exposed to ACPI via a shim driver (referred to as Surface ACPI Notify, or
SAN), translating ACPI generic serial bus write-/read-accesses to SAM
requests. On 7th generation devices, this additional layer is gone and these
devices require a driver hooking directly into the SAM interface. Equally,
on newer generations, less devices are declared in ACPI, making them a bit
harder to discover and requiring us to hard-code a sort of device registry.
Due to this, a SSAM bus and subsystem with client devices
(:c:type:`struct ssam_device <ssam_device>`) has been implemented.
Communication
=============
The type of communication interface between host and EC depends on the
generation of the Surface device. On 4th generation devices, host and EC
communicate via HID, specifically using a HID-over-I2C device, whereas on
5th and later generations, communication takes place via a USART serial
device. In accordance to the drivers found on other operating systems, we
refer to the serial device and its driver as Surface Serial Hub (SSH). When
needed, we differentiate between both types of SAM by referring to them as
SAM-over-SSH and SAM-over-HID.
Currently, this subsystem only supports SAM-over-SSH. The SSH communication
interface is described in more detail below. The HID interface has not been
reverse engineered yet and it is, at the moment, unclear how many (and
which) concepts of the SSH interface detailed below can be transferred to
it.
Surface Serial Hub
------------------
As already elaborated above, the Surface Serial Hub (SSH) is the
communication interface for SAM on 5th- and all later-generation Surface
devices. On the highest level, communication can be separated into two main
types: Requests, messages sent from host to EC that may trigger a direct
response from the EC (explicitly associated with the request), and events
(sometimes also referred to as notifications), sent from EC to host without
being a direct response to a previous request. We may also refer to requests
without response as commands. In general, events need to be enabled via one
of multiple dedicated requests before they are sent by the EC.
See :doc:`ssh` for a more technical protocol documentation and
:doc:`internal` for an overview of the internal driver architecture.

344
Documentation/driver-api/surface_aggregator/ssh.rst Normal file
View File

@ -0,0 +1,344 @@
.. SPDX-License-Identifier: GPL-2.0+
.. |u8| replace:: :c:type:`u8 <u8>`
.. |u16| replace:: :c:type:`u16 <u16>`
.. |TYPE| replace:: ``TYPE``
.. |LEN| replace:: ``LEN``
.. |SEQ| replace:: ``SEQ``
.. |SYN| replace:: ``SYN``
.. |NAK| replace:: ``NAK``
.. |ACK| replace:: ``ACK``
.. |DATA| replace:: ``DATA``
.. |DATA_SEQ| replace:: ``DATA_SEQ``
.. |DATA_NSQ| replace:: ``DATA_NSQ``
.. |TC| replace:: ``TC``
.. |TID| replace:: ``TID``
.. |IID| replace:: ``IID``
.. |RQID| replace:: ``RQID``
.. |CID| replace:: ``CID``
===========================
Surface Serial Hub Protocol
===========================
The Surface Serial Hub (SSH) is the central communication interface for the
embedded Surface Aggregator Module controller (SAM or EC), found on newer
Surface generations. We will refer to this protocol and interface as
SAM-over-SSH, as opposed to SAM-over-HID for the older generations.
On Surface devices with SAM-over-SSH, SAM is connected to the host via UART
and defined in ACPI as device with ID ``MSHW0084``. On these devices,
significant functionality is provided via SAM, including access to battery
and power information and events, thermal read-outs and events, and many
more. For Surface Laptops, keyboard input is handled via HID directed
through SAM, on the Surface Laptop 3 and Surface Book 3 this also includes
touchpad input.
Note that the standard disclaimer for this subsystem also applies to this
document: All of this has been reverse-engineered and may thus be erroneous
and/or incomplete.
All CRCs used in the following are two-byte ``crc_ccitt_false(0xffff, ...)``.
All multi-byte values are little-endian, there is no implicit padding between
values.
SSH Packet Protocol: Definitions
================================
The fundamental communication unit of the SSH protocol is a frame
(:c:type:`struct ssh_frame <ssh_frame>`). A frame consists of the following
fields, packed together and in order:
.. flat-table:: SSH Frame
:widths: 1 1 4
:header-rows: 1
* - Field
- Type
- Description
* - |TYPE|
- |u8|
- Type identifier of the frame.
* - |LEN|
- |u16|
- Length of the payload associated with the frame.
* - |SEQ|
- |u8|
- Sequence ID (see explanation below).
Each frame structure is followed by a CRC over this structure. The CRC over
the frame structure (|TYPE|, |LEN|, and |SEQ| fields) is placed directly
after the frame structure and before the payload. The payload is followed by
its own CRC (over all payload bytes). If the payload is not present (i.e.
the frame has ``LEN=0``), the CRC of the payload is still present and will
evaluate to ``0xffff``. The |LEN| field does not include any of the CRCs, it
equals the number of bytes inbetween the CRC of the frame and the CRC of the
payload.
Additionally, the following fixed two-byte sequences are used:
.. flat-table:: SSH Byte Sequences
:widths: 1 1 4
:header-rows: 1
* - Name
- Value
- Description
* - |SYN|
- ``[0xAA, 0x55]``
- Synchronization bytes.
A message consists of |SYN|, followed by the frame (|TYPE|, |LEN|, |SEQ| and
CRC) and, if specified in the frame (i.e. ``LEN > 0``), payload bytes,
followed finally, regardless if the payload is present, the payload CRC. The
messages corresponding to an exchange are, in part, identified by having the
same sequence ID (|SEQ|), stored inside the frame (more on this in the next
section). The sequence ID is a wrapping counter.
A frame can have the following types
(:c:type:`enum ssh_frame_type <ssh_frame_type>`):
.. flat-table:: SSH Frame Types
:widths: 1 1 4
:header-rows: 1
* - Name
- Value
- Short Description
* - |NAK|
- ``0x04``
- Sent on error in previously received message.
* - |ACK|
- ``0x40``
- Sent to acknowledge receival of |DATA| frame.
* - |DATA_SEQ|
- ``0x80``
- Sent to transfer data. Sequenced.
* - |DATA_NSQ|
- ``0x00``
- Same as |DATA_SEQ|, but does not need to be ACKed.
Both |NAK|- and |ACK|-type frames are used to control flow of messages and
thus do not carry a payload. |DATA_SEQ|- and |DATA_NSQ|-type frames on the
other hand must carry a payload. The flow sequence and interaction of
different frame types will be described in more depth in the next section.
SSH Packet Protocol: Flow Sequence
==================================
Each exchange begins with |SYN|, followed by a |DATA_SEQ|- or
|DATA_NSQ|-type frame, followed by its CRC, payload, and payload CRC. In
case of a |DATA_NSQ|-type frame, the exchange is then finished. In case of a
|DATA_SEQ|-type frame, the receiving party has to acknowledge receival of
the frame by responding with a message containing an |ACK|-type frame with
the same sequence ID of the |DATA| frame. In other words, the sequence ID of
the |ACK| frame specifies the |DATA| frame to be acknowledged. In case of an
error, e.g. an invalid CRC, the receiving party responds with a message
containing an |NAK|-type frame. As the sequence ID of the previous data
frame, for which an error is indicated via the |NAK| frame, cannot be relied
upon, the sequence ID of the |NAK| frame should not be used and is set to
zero. After receival of an |NAK| frame, the sending party should re-send all
outstanding (non-ACKed) messages.
Sequence IDs are not synchronized between the two parties, meaning that they
are managed independently for each party. Identifying the messages
corresponding to a single exchange thus relies on the sequence ID as well as
the type of the message, and the context. Specifically, the sequence ID is
used to associate an ``ACK`` with its ``DATA_SEQ``-type frame, but not
``DATA_SEQ``- or ``DATA_NSQ``-type frames with other ``DATA``- type frames.
An example exchange might look like this:
::
tx: -- SYN FRAME(D) CRC(F) PAYLOAD CRC(P) -----------------------------
rx: ------------------------------------- SYN FRAME(A) CRC(F) CRC(P) --
where both frames have the same sequence ID (``SEQ``). Here, ``FRAME(D)``
indicates a |DATA_SEQ|-type frame, ``FRAME(A)`` an ``ACK``-type frame,
``CRC(F)`` the CRC over the previous frame, ``CRC(P)`` the CRC over the
previous payload. In case of an error, the exchange would look like this:
::
tx: -- SYN FRAME(D) CRC(F) PAYLOAD CRC(P) -----------------------------
rx: ------------------------------------- SYN FRAME(N) CRC(F) CRC(P) --
upon which the sender should re-send the message. ``FRAME(N)`` indicates an
|NAK|-type frame. Note that the sequence ID of the |NAK|-type frame is fixed
to zero. For |DATA_NSQ|-type frames, both exchanges are the same:
::
tx: -- SYN FRAME(DATA_NSQ) CRC(F) PAYLOAD CRC(P) ----------------------
rx: -------------------------------------------------------------------
Here, an error can be detected, but not corrected or indicated to the
sending party. These exchanges are symmetric, i.e. switching ``rx`` and
``tx`` results again in a valid exchange. Currently, no longer exchanges are
known.
Commands: Requests, Responses, and Events
=========================================
Commands are sent as payload inside a data frame. Currently, this is the
only known payload type of |DATA| frames, with a payload-type value of
``0x80`` (:c:type:`SSH_PLD_TYPE_CMD <ssh_payload_type>`).
The command-type payload (:c:type:`struct ssh_command <ssh_command>`)
consists of an eight-byte command structure, followed by optional and
variable length command data. The length of this optional data is derived
from the frame payload length given in the corresponding frame, i.e. it is
``frame.len - sizeof(struct ssh_command)``. The command struct contains the
following fields, packed together and in order:
.. flat-table:: SSH Command
:widths: 1 1 4
:header-rows: 1
* - Field
- Type
- Description
* - |TYPE|
- |u8|
- Type of the payload. For commands always ``0x80``.
* - |TC|
- |u8|
- Target category.
* - |TID| (out)
- |u8|
- Target ID for outgoing (host to EC) commands.
* - |TID| (in)
- |u8|
- Target ID for incoming (EC to host) commands.
* - |IID|
- |u8|
- Instance ID.
* - |RQID|
- |u16|
- Request ID.
* - |CID|
- |u8|
- Command ID.
The command struct and data, in general, does not contain any failure
detection mechanism (e.g. CRCs), this is solely done on the frame level.
Command-type payloads are used by the host to send commands and requests to
the EC as well as by the EC to send responses and events back to the host.
We differentiate between requests (sent by the host), responses (sent by the
EC in response to a request), and events (sent by the EC without a preceding
request).
Commands and events are uniquely identified by their target category
(``TC``) and command ID (``CID``). The target category specifies a general
category for the command (e.g. system in general, vs. battery and AC, vs.
temperature, and so on), while the command ID specifies the command inside
that category. Only the combination of |TC| + |CID| is unique. Additionally,
commands have an instance ID (``IID``), which is used to differentiate
between different sub-devices. For example ``TC=3`` ``CID=1`` is a
request to get the temperature on a thermal sensor, where |IID| specifies
the respective sensor. If the instance ID is not used, it should be set to
zero. If instance IDs are used, they, in general, start with a value of one,
whereas zero may be used for instance independent queries, if applicable. A
response to a request should have the same target category, command ID, and
instance ID as the corresponding request.
Responses are matched to their corresponding request via the request ID
(``RQID``) field. This is a 16 bit wrapping counter similar to the sequence
ID on the frames. Note that the sequence ID of the frames for a
request-response pair does not match. Only the request ID has to match.
Frame-protocol wise these are two separate exchanges, and may even be
separated, e.g. by an event being sent after the request but before the
response. Not all commands produce a response, and this is not detectable by
|TC| + |CID|. It is the responsibility of the issuing party to wait for a
response (or signal this to the communication framework, as is done in
SAN/ACPI via the ``SNC`` flag).
Events are identified by unique and reserved request IDs. These IDs should
not be used by the host when sending a new request. They are used on the
host to, first, detect events and, second, match them with a registered
event handler. Request IDs for events are chosen by the host and directed to
the EC when setting up and enabling an event source (via the
enable-event-source request). The EC then uses the specified request ID for
events sent from the respective source. Note that an event should still be
identified by its target category, command ID, and, if applicable, instance
ID, as a single event source can send multiple different event types. In
general, however, a single target category should map to a single reserved
event request ID.
Furthermore, requests, responses, and events have an associated target ID
(``TID``). This target ID is split into output (host to EC) and input (EC to
host) fields, with the respecting other field (e.g. output field on incoming
messages) set to zero. Two ``TID`` values are known: Primary (``0x01``) and
secondary (``0x02``). In general, the response to a request should have the
same ``TID`` value, however, the field (output vs. input) should be used in
accordance to the direction in which the response is sent (i.e. on the input
field, as responses are generally sent from the EC to the host).
Note that, even though requests and events should be uniquely identifiable
by target category and command ID alone, the EC may require specific
target ID and instance ID values to accept a command. A command that is
accepted for ``TID=1``, for example, may not be accepted for ``TID=2``
and vice versa.
Limitations and Observations
============================
The protocol can, in theory, handle up to ``U8_MAX`` frames in parallel,
with up to ``U16_MAX`` pending requests (neglecting request IDs reserved for
events). In practice, however, this is more limited. From our testing
(although via a python and thus a user-space program), it seems that the EC
can handle up to four requests (mostly) reliably in parallel at a certain
time. With five or more requests in parallel, consistent discarding of
commands (ACKed frame but no command response) has been observed. For five
simultaneous commands, this reproducibly resulted in one command being
dropped and four commands being handled.
However, it has also been noted that, even with three requests in parallel,
occasional frame drops happen. Apart from this, with a limit of three
pending requests, no dropped commands (i.e. command being dropped but frame
carrying command being ACKed) have been observed. In any case, frames (and
possibly also commands) should be re-sent by the host if a certain timeout
is exceeded. This is done by the EC for frames with a timeout of one second,
up to two re-tries (i.e. three transmissions in total). The limit of
re-tries also applies to received NAKs, and, in a worst case scenario, can
lead to entire messages being dropped.
While this also seems to work fine for pending data frames as long as no
transmission failures occur, implementation and handling of these seems to
depend on the assumption that there is only one non-acknowledged data frame.
In particular, the detection of repeated frames relies on the last sequence
number. This means that, if a frame that has been successfully received by
the EC is sent again, e.g. due to the host not receiving an |ACK|, the EC
will only detect this if it has the sequence ID of the last frame received
by the EC. As an example: Sending two frames with ``SEQ=0`` and ``SEQ=1``
followed by a repetition of ``SEQ=0`` will not detect the second ``SEQ=0``
frame as such, and thus execute the command in this frame each time it has
been received, i.e. twice in this example. Sending ``SEQ=0``, ``SEQ=1`` and
then repeating ``SEQ=1`` will detect the second ``SEQ=1`` as repetition of
the first one and ignore it, thus executing the contained command only once.
In conclusion, this suggests a limit of at most one pending un-ACKed frame
(per party, effectively leading to synchronous communication regarding
frames) and at most three pending commands. The limit to synchronous frame
transfers seems to be consistent with behavior observed on Windows.

1
MAINTAINERS
View File

@ -11818,6 +11818,7 @@ M: Maximilian Luz <luzmaximilian@gmail.com>
S: Maintained
W: https://github.com/linux-surface/surface-aggregator-module
C: irc://chat.freenode.net/##linux-surface
F: Documentation/driver-api/surface_aggregator/
F: drivers/platform/surface/aggregator/
F: include/linux/surface_aggregator/