Messages in this thread | | | Date | Sat, 23 Jun 2007 14:49:47 +0200 | From | "Kay Sievers" <> | Subject | Re: Rules on how to use sysfs in userspace programs |
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On 6/22/07, Rob Landley <rob@landley.net> 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.
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