Re: Rules on how to use sysfs in userspace programs

[Kay Sievers]

On 6/22/07, Rob Landley <rob@xxxxxxxxxxx> wrote:
> On Friday 08 June 2007 16:36:37 Greg KH wrote:
> > Over time there have been a number of problems when sysfs has changed in
> > "unexpected" ways.  Here's a document that Kay wrote a while ago that
> > I'd like to add to the kernel Documentation directory to help userspace
> > programmers out.
> >
> > Any comments or critique of this is greatly appreciated.
>
> Still catching up from my laptop dying.
>
> I find the explanation of /sys/subsystem almost unintelligible.  What will the
> new one actually look like?

"It is planned to merge all three classification-directories into one
place at /sys/subsystem/,  following the current layout of the
bus-directories."

Means that /sys/subsystem/ will have a devices/ directory, full of
symlinks to the devices, all in a flat list. Subsytem-global attribute
files/directories are not mixed with the devices in the same directory
like in /sys/class, it will also not contain any hierarchy like the
layout of /sys/block.

> If I want to find all block devices in the system, it looks like I should now
> look at /sys/subsystem/block.

Yes, in this order (if you want to use it, but /sys/block will still be there):
 /sys/subsytem/block/devices/*
 /sys/class/block/*
 /sys/block/*/*

> (And "subsystem" is not a variable here but
> the actual directory name?  I presume it moved for Feng Shui reasons.)

"one place at /sys/subsystem/"

Yes, it will be all pretty consistent, the event-environment contains
$SUBSYSTEM, we will  have /sys/subsystem/$SUBSYSTEM/devices/ directory
and at every device a symlink named "subsystem" pointing back to the
/sys/subsystem/$SUBSYSTEM/ directory.

> If I want to find char devices, where do I look?  /sys/subsystem...  char?
> class?  Is a char device now anything under /sys/subsystem that is _not_
> in /sys/subsystem/block?  (Minus bus devices?)  Is there a specific directory
> for these?

If /sys/subsystem exists, just look at /sys/subsystem/*/devices/*, you
will find every kernel device here, with exactly the logic to access
it. Every device with a "dev" file, it is a char device, unless
$SUBYSTEM=="block".

If /sys/subsystem/ doesn't exist, you have to search all through
/sys/bus/, /sys/class/, /sys/block/, every directory with completely
different access pattern to find your device. You may want to look at
the udev code, it's all implemented there:
http://git.kernel.org/?p=linux/hotplug/udev.git;a=blob;f=udevtrigger.c;hb=HEAD#l498

> This document is highly polluted with what NOT to do.

Yeah, that's sysfs. It exports all the useless kernel implementation
details, so that _what_not_to_do_ is the biggest problem we have. :)
There is much too much internal stuff available here, that never can
be kept stable in the usual sense, as long as we allow to change
kernel/driver internals at the same time.

> I'm looking for a
> clear "what SHOULD I do", and it takes some wading to find it.  (Historical
> cruft about what not to do is potentially a separate document, it's not
> useful for people learning this stuff now who weren't playing with the legacy
> mechanisms.)  The description of /sys/subsystem spends so much time talking
> about old legacy issues it never gives a clear description of the new way of
> doing things, which is theoretically what this document is about...

It was a first cut, I did months ago, and sure, it needs some work.

Thanks,
Kay
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