[Intel-gfx] [PATCH 3/4] drm/i915: expose engine availability through sysfs

[Chris Wilson]

Quoting Chris Wilson (2017-11-28 20:56:23)
> Quoting Tvrtko Ursulin (2017-11-28 18:17:54)
> > 
> > On 20/11/2017 12:23, Lionel Landwerlin wrote:
> > > This enables userspace to discover the engines available on the GPU.
> > > Here is the layout on a Skylake GT4:
> > > 
> > > /sys/devices/pci0000:00/0000:00:02.0/drm/card0/gt
> > 
> > On this one I think Joonas had a concern that it is difficult for
> > userspace to get to the sysfs root from the drm file descriptor.
> > 
> > Lionel pointed out that for master nodes it is quite easy:
> > 
> > fstat(fd, &st);
> > sprintf(sysfs_root, "/sys/dev/char/%u:%u", major(st.st_rdev), minor(st.st_rdev));
> > 
> > For render nodes it is trickier in a way that they would have to
> > additional resolve an unknown master drm card number. For instance:
> > 
> > /sys/dev/char/%u:%u/device/drm/cardX
> > 
> > Where the "X" is unknown.
> > 
> > But, is it even an use case for render nodes to try and get to the
> > sysfs root of the master card? Are they allowed to do so?
> 
> Yes. Mesa uses render nodes, and mesa needs the topology for its
> performance monitoring api.

So /sys/dev/char/226:128/ does not link to /sys/dev/char/226:0/. Maybe
we should just add a card symlink from each minor back to our sysfs
root? That seems doable.

Then we just need open("/sys/dev/char/%u:%u/card");
-Chris