On 28/11/2017 21:39, Chris Wilson wrote:
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");
There is a link via what I wrote above -
"/sys/dev/char/%u:%u/device/drm/cardX", but the "X" part is cumbersome.
Adding a link with a constant name would indeed be better, but I wasn't
sure if it is conceptually allowed and even forgot to mention it.
Regards,
Tvrtko
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