Re: work-around for video4linux sysfs

[Kay Sievers <kay.sievers@vrfy.org>]

On Thu, 2007-08-02 at 17:39 -0500, Linas Vepstas wrote:
> On Fri, Aug 03, 2007 at 12:30:12AM +0200, Kay Sievers wrote:
> > On Thu, 2007-08-02 at 07:05 -0700, Kees Cook wrote:
> > > 
> > > pci-0000:00:0e.0-scsi-0:0:0:0-part1
> > >                   ^driver
> > >                           ^simple enumeration
> > 
> > It's the subsystem not the driver. And it's not an enumeration based on
> > discovery order, it's the PCI slot number which doesn't change, also
> 
> I haven't tried recently, but in the "good old days", I used to
> get bitten hard every time I shuffled around the PCI cards from
> one slot to another, to make room for one more.

"Shuffling around" is absolutly expected to change "by-path" links,
that's actually the correct and expected behavior. :)
There are by-id links, which are independent of the
path/port/slot/numbers. For block devices there are by-uuid and
by-label, which don't change by moving hardware around.

> > SCSI uses stable interface and port numbers here. 
> 
> Hehe. SCSI LUN's can change when one is adding and removing failed 
> disks, or shuffling around so that one can re-cable.  That was
> another source of heartburn.

Same here, by-path must change if the path to the hardware changes.
Depending on the setup, these links are more or less useful than the
links based on the content or properties owned by the hardware.

> The only viable solution is to work off some unique ID on the 
> device (e.g. a MAC address on ethernet).

The MAC is fine for most of the simple setups, yes. But there are lots
of cases where they just don't work. There are whole architectures in
the kernel which only use random MAC addresses and you need to fall back
to the path to the device.

Oh, and there are a lot of properties on other subsystems which can be
used to create reliable device names. Depending on the actual hardware
or the setup, there is no general rule which category of links is more
viable than the other, just look what /dev/disk offers, they are all
needed.

Kay


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