[PATCH v6] drm/ioctl: Add a ioctl to set and get a label on GEM objects

[Eric Anholt]

On Fri, May 29, 2020 at 6:44 AM Rohan Garg <rohan.garg at collabora.com> wrote:
>
> Hey Eric!
>
> On jueves, 28 de mayo de 2020 20:45:24 (CEST) Eric Anholt wrote:
> > On Thu, May 28, 2020 at 10:06 AM Rohan Garg <rohan.garg at collabora.com>
> wrote:
> > > DRM_IOCTL_HANDLE_SET_LABEL lets you label buffers associated
> > > with a handle, making it easier to debug issues in userspace
> > > applications.
> > >
> > > DRM_IOCTL_HANDLE_GET_LABEL lets you read the label associated
> > > with a buffer.
> > >
> > > Changes in v2:
> > >   - Hoist the IOCTL up into the drm_driver framework
> > >
> > > Changes in v3:
> > >   - Introduce a drm_gem_set_label for drivers to use internally
> > >
> > >     in order to label a GEM object
> > >
> > >   - Hoist string copying up into the IOCTL
> > >   - Fix documentation
> > >   - Move actual gem labeling into drm_gem_adopt_label
> > >
> > > Changes in v4:
> > >   - Refactor IOCTL call to only perform string duplication and move
> > >
> > >     all gem lookup logic into GEM specific call
> > >
> > > Changes in v5:
> > >   - Fix issues pointed out by kbuild test robot
> > >   - Cleanup minor issues around kfree and out/err labels
> > >   - Fixed API documentation issues
> > >   - Rename to DRM_IOCTL_HANDLE_SET_LABEL
> > >   - Introduce a DRM_IOCTL_HANDLE_GET_LABEL to read labels
> > >   - Added some documentation for consumers of this IOCTL
> > >   - Ensure that label's cannot be longer than PAGE_SIZE
> > >   - Set a default label value
> > >   - Drop useless warning
> > >   - Properly return length of label to userspace even if
> > >
> > >     userspace did not allocate memory for label.
> > >
> > > Changes in v6:
> > >   - Wrap code to make better use of 80 char limit
> > >   - Drop redundant copies of the label
> > >   - Protect concurrent access to labels
> > >   - Improved documentation
> > >   - Refactor setter/getter ioctl's to be static
> > >   - Return EINVAL when label length > PAGE_SIZE
> > >   - Simplify code by calling the default GEM label'ing
> > >
> > >     function for all drivers using GEM
> > >
> > >   - Do not error out when fetching empty labels
> > >   - Refactor flags to the u32 type and add documentation
> > >   - Refactor ioctls to use correct DRM_IOCTL{R,W,WR} macros
> > >   - Return length of copied label to userspace
> > >
> > > Signed-off-by: Rohan Garg <rohan.garg at collabora.com>
> >
> > I don't think we should land this until it actually does something
> > with the label, that feels out of the spirit of our uapi merge rules.
> > I would suggest looking at dma_buf_set_name(), which would produce
> > useful output in debugfs's /dma_buf/buf_info.  But also presumably you
> > have something in panfrost using this?
> >
>
> My current short term plan is to hook up glLabel to the labeling functionality
> in order for userspace applications to debug exactly which buffer objects
> could be causing faults in the kernel for speedier debugging.

So, something like "when a fault happens, dump/capture names of nearby
objects"?  That would certainly be nice to have!

> The more long term plan is to label each buffer with a unique id that we can
> correlate to the GL calls being flushed in order to be able to profile (a set
> of)  GL calls on various platforms in order to aid driver developers with
> performance work. This could be something that we corelate on the userspace
> side with the help of perfetto by using this [1] patch that emits ftrace
> events.

I'm curious how BO names are part of profiling?  Do you have the
ability to sample instruction IP and use that to figure out where time
is spent in shaders, or something?