On 6/22/07, Rob Landley <[email protected]> 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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
