systemd-stable/docs/overview

Instead of heading in on another "proposal" document, I thought I'd send out 
this email describing ideas I've had about udev - thanks to the comments I've 
received. The idea is starting to mushroom a bit and I'd like to get people's 
comments before I go further down the path.

As I see it, we've got a couple goals for udev:

1) dynamic replacement for /dev
2) device naming
3) API to access info about current system devices

I'd like to split these goals into separate subsystems:

1) udev - dynamic replacement for /dev
2) namedev - device naming
3) libsysfs - a standard library for accessing device information on the 
system.

Udev
------

Udev will be responsible for responding to /sbin/hotplug on device events. It 
will receive the device class information along with device's sysfs 
directory. Udev will call the name_device function from the naming device 
subsystem with that information and receive a unique device name in return. 
Udev will then query sysfs through the libsysfs for specific device 
information required for creating the /dev node like major and minor number. 
Once it has the important information, udev will create a /dev entry for the 
device, add the device to the in memory table of current devices, and send 
notification of the successful event. On a remove call, udev will remove the 
/dev entry, remove the device from the in memory table, and send 
notification. 

Udev will consist of a command udev - to be called from /sbin/hotplug. It will 
require the in memory dynamic database/table for keeping track of current 
system devices, and a library of routines for accessing that database/table. 
Udev will not care about "how" devices are named, that will be separated into 
the device naming subsystem. It's presented a common device naming API by the 
device naming subsystem to use for naming devices.

namedev
----------

From comments Martin has made, I've decided to push out the device naming part 
of udev into its own "subsystem". The reason is to make this as flexible and 
pluggable as possible. The device naming subsystem, or namedev, will present 
a standard interface for udev to call for naming a particular device. Under 
that interface, system administrators can plug in their own methods for 
device naming. 

We would provide a default naming scheme. The first prototype implementation 
could simply take the sysfs directory passed in with the device name 
function, query sysfs for the major and minor numbers, and then look up in a 
static device name mapping file the name of the device. The static device 
naming file could look just like devices.txt in the Linux kernel's 
Documentation directory. Obviously, this isn't a great implementation because 
eventually we'd like major an minor numbers to be dynamic.

The default naming scheme in the future would have a set of policies to go 
through, these were given to me by Greg. The device naming subsystem would 
get the sysfs directory of the to be named device and would use the following 
information in order to map the device's name:

1) Label info - like SCSI's UUID
2) Bus Device Number
3) Topology on Bus
4) Kernel Name - DEFAULT

System administrators could use the default naming system or enterprise 
computing environments could plug in their Universal Unique Identifier (UUID) 
policies. The idea is to make the device naming as flexible and pluggable as 
possible.

The device naming subsystem would require accessing sysfs for device 
information. It will receive the device's sysfs directory in the call from 
udev and use it to get more information to determine naming. The namedev 
subsystem will include a standard naming API for udev to use. The default 
naming scheme will include a set of functions and a static device naming 
file, which will reside in /etc or /var. 

libsysfs
--------

Greg may object, but I believe there's a need for a common API to access 
device information in sysfs. The device naming subsystem and the udev 
subsystem need to take the sysfs directory path and query device information. 
Instead of copying code so each one will have to readdir, etc., I've decided 
to split out the sysfs calls into a separate library that will sit atop 
sysfs. Sysfs callbacks aren't standard across devices, I beleive this is 
another reason for creating a common and standard library interface for 
querying device information. 

Another reason for libsysfs is it satisfies requirements the LTC RAS team has 
for getting current system device information. Rather than keeping tons of 
information in udev's in memory database, or even querying that database for 
the sysfs directory that will require storing extra reference info in memory, 
I've decided the RAS requirements can be fulfilled with a library atop sysfs. 
Sysfs contains devices currently on the system. 

Applications like the Error Log Analysis piece, for example, can query the 
sysfs library for device information. ELA gets specific information in an 
error message thanks to the dev_* and soon to be proposed netdev_* macros. 
One goal of the ELA is to gather as much information about an erroring device 
so service engineers and administrators can diagnose the problem. The ELA 
will get an error message with the bus id and driver name of the device. It 
will then need to query sysfs for other VPD information. 

I've used syfs in the name of libsysfs for a reason, I believe sysfs will be 
the device tree to use in the future. Until all VPD info is in sysfs, the 
library could also make use of /proc, sginfo, and other sources for device 
information under the covers so ELA and other applications don' t need to all 
have that knowledge. 


I'd like to know what everyone thinks about my proposal to split this all up 
into three separate subsystems. All comments are welcome.

Thanks,

Dan