[Spice-devel] [RFC linux vdagent] Add POC for getting xrandr output from monitor ID

[Lukáš Hrázký]

Hi,

On Tue, 2018-11-13 at 09:04 +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> > To look up a PCI device by address, I could use the /sys filesystem. So
> > for pci/0000/02.0, I would translate the domain 0000 into the
> > filesystem path '/sys/devices/pci0000:00/' assuming bus '00'?  And then
> > look inside that directory for the the path '0000:00:02.0'. And then
> > look for a path named 'drm' within that directory to determine whether
> > it's a drm card or not? Is the existence of the 'drm' directory the
> > thing that we should use to determine whether this is a drm card? 
> 
> Typically yes, except virtio-gpu which has an additional virtio level,
> i.e. /sys/devices/pci0000:00/0000:00:02.0/virtio<n>/drm.

Why the exception? Doesn't make this check look that great and
reliable...

> > Or a slightly more complicated example (based on my laptop): Let's look
> > for this device:
> > 
> > 02:00.0 3D controller: NVIDIA Corporation GM108M [GeForce 940MX] (rev
> > a2)
> > 
> > So it's device 00.0 on bus 02. Here's the first part of lspci -t:
> > -[0000:00]-+-00.0
> >            +-01.0-[01]--
> >            +-01.2-[02]----00.0
> > 
> > So it's attached to the PCI bridge at 01.2. Since the bus number ('02')
> > is not a part of our device address format, we would only be provided
> > the following address: pci/0000/01.2/00.0
> > 
> > From this we can look for the PCI bridge at path
> > /sys/devices/pci0000:00/0000:00:01.2. Within this directory, we can
> > look for a path of ... it's unclear because we're not provided the bus
> > number, so we can't necessarily construct the path '0000:02:00.0' that
> > represents this device. We could simply look for files within the
> > directory that end with '00.0' and just assume that the one we find is
> > the correct one. But that doesn't seem cleaner and more elegant to me.
> 
> You can translate the path into a sysfs shell globbing pattern, then
> feed that into glob (see "man glob").
> 
> For "pci/0000/01.2/00.0" the glob pattern would be
> "/sys/devices/pci0000:*/*:01.2/*:00.0"

That's what I had in mind.

Cheers,
Lukas

> HTH,
>   Gerd
>