sysfs question: how to map major:minor to name under /sys

[Neil Brown <neilb@suse.de>]

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).

 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?

 The particular case that I am interested in involves md.
 In this case I can find the right /sys/block/mdX entry easily enough
 because I *know* how the names are generated.  However when a block
 device is added to an array, it gets an entry like
    /sys/block/md4/md/dev-sda
 I would like to be able to easily find that given just the
 information that 8:0 was added.
 If the above directory of symlinks existed, I could readlink the
 relevant entry, take the basename, and add "md/dev-" to front of
 that. 

 Comments?

NeilBrown
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