Re: [PATCH/RFC] allow mapping from block-device-file to sysfs entry.

[Neil Brown <neilb@suse.de>]

On Monday August 6, arjan@infradead.org wrote:
> On Tue, 2007-08-07 at 16:07 +1000, Neil Brown wrote:
> > Suppose that in a program I have an open file descriptor for a device,
> > and I want to find the /sys/block information for this device.
> > There is currently no direct way to do this.  I need to read
> >    /sys/block/*/dev, /sys/block/*/*/dev
> > and match major/minor numbers with the result from fstat.
> > 
> > I would like a more direct mechanism.
> > 
> > The following patch is a proposal for such a mechanism.
> > 
> > It provides an 'ioctl' which returns then 'name' of the device, as
> > generated by bdevname.  This is the same name that is used to create
> > entries in sysfs.
> > For a partition of a device, it returns 'device/partition'.
> 
> 
> how about returning the entire path relative to the start of sysfs? That
> way, if things move or something you're tolerant against that....

That makes a lot of sense.
So it would return "block/sda/sda1" now, but one day that might change
to "class/block/sda/sda1" or some-such.

> 
> (I'd not be against making this a generic IOCTL for every device, a
> SYSFSLOCATION kind of ioctl... it's by no means block specific...)
> 

That too seems very sensible.  Only it's harder to choose a 'generic'
ioctl request number than to choose a block-specific one :-)

We would also need a somewhat longer buffer.  The longest
/sys/**/dev
path on my test machine is

/sys/devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1:1.1/usb_endpoint/usbdev2.2_ep82/dev

So at least 80 chars.  Probably 256 would do....

#define SYSFSLOCATION _IOR(0, 0xff, char[256])
??

Thanks,
NeilBrown