2255402040
While splitting the document up, the headings "shifted" from what pandoc
generated. Use the following order for headings for consistency:
==============
Document title
==============
First
=====
Second
------
Third
~~~~~
Leave the lower level headings as they are; I think those are less
important. Although RST doesn't mandate a specific order ("Rather than
imposing a fixed number and order of section title adornment styles, the
order enforced will be the order as encountered."), having the higher
levels the same overall makes it easier to follow the documents.
[I'm sort of kind of writing the recommendation for docs-next in the
mean time, but this order seems sensible, and is what I'm proposing.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/126f42734defac6cbb8496a481d58db7b38461dd.1466506505.git.jani.nikula@intel.com
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===================
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Userland interfaces
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===================
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The DRM core exports several interfaces to applications, generally
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intended to be used through corresponding libdrm wrapper functions. In
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addition, drivers export device-specific interfaces for use by userspace
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drivers & device-aware applications through ioctls and sysfs files.
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External interfaces include: memory mapping, context management, DMA
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operations, AGP management, vblank control, fence management, memory
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management, and output management.
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Cover generic ioctls and sysfs layout here. We only need high-level
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info, since man pages should cover the rest.
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Render nodes
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============
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DRM core provides multiple character-devices for user-space to use.
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Depending on which device is opened, user-space can perform a different
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set of operations (mainly ioctls). The primary node is always created
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and called card<num>. Additionally, a currently unused control node,
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called controlD<num> is also created. The primary node provides all
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legacy operations and historically was the only interface used by
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userspace. With KMS, the control node was introduced. However, the
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planned KMS control interface has never been written and so the control
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node stays unused to date.
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With the increased use of offscreen renderers and GPGPU applications,
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clients no longer require running compositors or graphics servers to
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make use of a GPU. But the DRM API required unprivileged clients to
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authenticate to a DRM-Master prior to getting GPU access. To avoid this
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step and to grant clients GPU access without authenticating, render
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nodes were introduced. Render nodes solely serve render clients, that
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is, no modesetting or privileged ioctls can be issued on render nodes.
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Only non-global rendering commands are allowed. If a driver supports
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render nodes, it must advertise it via the DRIVER_RENDER DRM driver
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capability. If not supported, the primary node must be used for render
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clients together with the legacy drmAuth authentication procedure.
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If a driver advertises render node support, DRM core will create a
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separate render node called renderD<num>. There will be one render node
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per device. No ioctls except PRIME-related ioctls will be allowed on
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this node. Especially GEM_OPEN will be explicitly prohibited. Render
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nodes are designed to avoid the buffer-leaks, which occur if clients
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guess the flink names or mmap offsets on the legacy interface.
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Additionally to this basic interface, drivers must mark their
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driver-dependent render-only ioctls as DRM_RENDER_ALLOW so render
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clients can use them. Driver authors must be careful not to allow any
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privileged ioctls on render nodes.
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With render nodes, user-space can now control access to the render node
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via basic file-system access-modes. A running graphics server which
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authenticates clients on the privileged primary/legacy node is no longer
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required. Instead, a client can open the render node and is immediately
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granted GPU access. Communication between clients (or servers) is done
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via PRIME. FLINK from render node to legacy node is not supported. New
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clients must not use the insecure FLINK interface.
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Besides dropping all modeset/global ioctls, render nodes also drop the
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DRM-Master concept. There is no reason to associate render clients with
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a DRM-Master as they are independent of any graphics server. Besides,
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they must work without any running master, anyway. Drivers must be able
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to run without a master object if they support render nodes. If, on the
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other hand, a driver requires shared state between clients which is
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visible to user-space and accessible beyond open-file boundaries, they
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cannot support render nodes.
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VBlank event handling
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=====================
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The DRM core exposes two vertical blank related ioctls:
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DRM_IOCTL_WAIT_VBLANK
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This takes a struct drm_wait_vblank structure as its argument, and
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it is used to block or request a signal when a specified vblank
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event occurs.
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DRM_IOCTL_MODESET_CTL
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This was only used for user-mode-settind drivers around modesetting
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changes to allow the kernel to update the vblank interrupt after
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mode setting, since on many devices the vertical blank counter is
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reset to 0 at some point during modeset. Modern drivers should not
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call this any more since with kernel mode setting it is a no-op.
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This second part of the GPU Driver Developer's Guide documents driver
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code, implementation details and also all the driver-specific userspace
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interfaces. Especially since all hardware-acceleration interfaces to
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userspace are driver specific for efficiency and other reasons these
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interfaces can be rather substantial. Hence every driver has its own
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chapter.
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