[Intel-gfx] [PATCH v2] drm/i915/gt: Register the migrate contexts with their engines

[Thomas Hellström]

On Thu, 2021-08-26 at 14:04 +0100, Tvrtko Ursulin wrote:
> 
> On 26/08/2021 11:45, Thomas Hellström wrote:
> > Pinned contexts, like the migrate contexts need reset after resume
> > since their context image may have been lost. Also the GuC needs to
> > register pinned contexts.
> 
> So kernel context can get corrupt because we park the GPU with it 
> active. Blitter context for a different reason - which is that it is 
> used to copy itself over to smem, no?
> 
> If that is correct, then why bother copying the blitter context in
> the 
> first place and not just always re-create it on resume?
> 
> That would be along the lines of marking the backing store as
> "dontneed" 
> (however the exact mechanics of that look these days) so suspend can 
> skip them.

I think that is marking the object with I915_BO_ALLOC_VOLATILE. However
I assume this follows the rule of the internal backend objects:
Contents are valid while pinned (or locked), and these images are
indeed pinned on suspend so we need to come up with something else.
Perhaps I915_BO_ALLOC_PM_NOSAVE for the context images (and engine
status pages?) I915_BO_ALLOC_PM_MEMCPY for the migrate vm pagetables
only. The latter will come in handy also for supporting small apertures
where we need to pin these in the mappable area.

> 
> > Add a list to struct intel_engine_cs where we add all pinned
> > contexts on
> > creation, and traverse that list at resume time to reset the pinned
> > contexts.
> > 
> > This fixes the kms_pipe_crc_basic at suspend-read-crc-pipe-a selftest
> > for now,
> > but proper LMEM backup / restore is needed for full suspend
> > functionality.
> > However, note that even with full LMEM backup / restore it may be
> > desirable to keep the reset since backing up the migrate context
> > images
> > must happen using memcpy() after the migrate context has become
> > inactive,
> > and for performance- and other reasons we want to avoid memcpy()
> > from
> > LMEM.
> 
> Hm I guess this talks about the issue - so are these images migrated
> at 
> all today or not?

My current WIP backs them up. But with something like the above flags,
that's easily changed. Suggestions welcome.

/Thomas