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

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Date:	Mon, 17 Dec 2007 09:24:39 +0100
From:	Kay Sievers <kay.sievers@...y.org>
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	linux-kernel@...r.kernel.org, Greg KH <greg@...ah.com>
Subject: Re: /sys/block [was: [PATCH 007 of 7] md: Get name for block
	device in sysfs]

On Mon, 2007-12-17 at 08:29 +0300, Michael Tokarev wrote:
> 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?

By its subsystem value (block), from the symlink, from the environment,
or from $1.

> 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....

No need.

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

$SUBSYSTEM == "block"

> 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).

/sys/block is still there and contains symlinks. And all this happens
only for !SYSFS_DEPRECATED.

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

Unification. Block devices are "struct devices", use a class, use the
common driver core code instead of their own, show up in the tree, and
can be parents for other devices.

Kay

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