linux-kernel - /sys/block [was: [PATCH 007 of 7] md: Get name for block device in sysfs]

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Date:	Mon, 17 Dec 2007 08:29:11 +0300
From:	Michael Tokarev <mjt@....msk.ru>
To:	Kay Sievers <kay.sievers@...y.org>
CC:	linux-kernel@...r.kernel.org, Greg KH <greg@...ah.com>
Subject: /sys/block [was: [PATCH 007 of 7] md: Get name for block device in
 sysfs]

Kay Sievers wrote:
> On Mon, 2007-12-17 at 09:43 +1100, Neil Brown wrote:
>> On Saturday December 15, kay.sievers@...y.org wrote:
>>> On Dec 14, 2007 7:26 AM, NeilBrown <neilb@...e.de> wrote:
>>>> Given an fd on a block device, returns a string like
>>>>
>>>>         /block/sda/sda1
>>>>
>>>> which can be used to find related information in /sys.
>> ....
>>> As pointed out to when you came up with the idea, we can't do this. A devpath
>>> is a path to the device and will not necessarily start with "/block" for block
>>> devices. It may start with "/devices" and can be much longer than
>>> BDEVNAME_SIZE*2  + 10.
>> When you say "will not necessarily" can I take that to mean that it
>> currently does, but it might (will) change??
> 
> It's in -mm. The devpath for all block devices, like for all other
> devices, will start with /devices/* if !SYSFS_DEPRECATED.

This is the second time I come across this (planned?) change, and for
the second time I can't understand it.

How to distinguish char devices from block devices in sysfs?
Is the only way to read a symlink `subsystem' in the device
directory?

For now, I've a shell code (used heavily in numerous places),
which looks like this:

  function makedev() {
    ...
    case $DEVPATH in
      /block/*) TYPE=b ;;
      *) TYPE=c ;;
    esac
    ...
    mknod /dev/$DEV $TYPE $MAJOR $MINOR
  }

The only external process invocation in there is mknod, all
the rest is done using pure shell constructs.  Is it really
necessary to spawn another process just to read a symlink
now?  It will be almost 2 times slower....

(Sure thing this may be rewritten in C, but using shell it's
MUCH easier to customize if necessary.)

Also, /sys/block/ directory is very easy to use currently, --
unlike other /sys/ stuff which is way too deep and often
placed in unknown/unexpected places (and /sys/class/ and
/sys/bus/ directories are changing all the time).

What's the benefit of moving things from /sys/block/ to
/sys/devices/ ?

Thanks.

/mjt
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