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From: Neil Brown <neilb@suse.de>
To: Greg KH <greg@kroah.com>
Date: Fri, 16 Dec 2005 14:36:19 +1100
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Cc: linux-kernel@vger.kernel.org
Subject: Re: sysfs question:  how to map major:minor to name under /sys
In-Reply-To: message from Greg KH on Thursday December 15
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On Thursday December 15, greg@kroah.com wrote:
> On Thu, Dec 15, 2005 at 02:14:06PM +1100, Neil Brown wrote:
> > 
> > Hi,
> >  I have a question about sysfs related usage.
> > 
> >  Suppose I have a major:minor number for a block device - maybe from
> >  fstat of a filedescriptor I was given, or stat of a name in /dev.
> >  How do I find the directory in /sys/block that contains relevant
> >  information? 
> > 
> >  It seems to me that there is no direct way, and maybe there should
> >  be. (I can do a find of 'dev' file and compare, which is fine in a
> >  one-off shell script, but sub-optimal in general).
> 
> So you want this info from userspace, not from within the kernel,
> right?

Right.

> 
> >  The most obvious solution would be to have a directory somewhere full
> >  of symlinks:
> >         /sys/block_dev/8:0 -> ../block/sda
> >  or whatever.
> >  Is this reasonable?  Should I try it?
> 
> It seems a bit md specific to add a lot of kernel code for something
> that can be solved with a userspace shell script :)

I don't see it as md specific.  
Suppose I want to change the IO scheduler under the filesystem /foo.
I look that up in mtab and find it is mounted on /dev/blah.
To find the corresponding /sys/block entry I have to search.  That's
fine if I only have a few block devices.  But I keep hearing storing
of people with thousands.  Having to search just feels clumsy.

> 
> But if you want to try it, use a class, and a class_device for this, not
> raw kobjects.  It should be a bit easier that way.

Well, maybe it'll be a good excuse to learn more about 'class' and
'class_device' anyway.
Thanks,
NeilBrown