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

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Thu, 2021-08-26 at 14:44 +0200, Daniel Vetter wrote:
> On Thu, Aug 26, 2021 at 12:45:14PM +0200, 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.
> > 
> > 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@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.
> > 
> > Also traverse the list at guc_init_lrc_mapping() calling
> > guc_kernel_context_pin() for the pinned contexts, like is already
> > done
> > for the kernel context.
> > 
> > v2:
> > - Don't reset the contexts on each __engine_unpark() but rather at
> >   resume time (Chris Wilson).
> > 
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> > Cc: Matthew Auld <matthew.auld@intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Brost Matthew <matthew.brost@intel.com>
> > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> 
> I guess it got lost, but I few weeks ago I stumbled over this and
> wondered
> why we're even setting up a separate context or at least why a
> separate vm
> compared to the gt->vm we have already?
> 
> Even on chips with bazillions of copy engines the plan is that we
> only
> reserve a single one for kernel migrations, so there's not really a
> need
> for quite this much generality I think. Maybe check with Jon
> Bloomfield on
> this.

Are you referring to the generality of the migration code itself or to
the generality of using a list in this patch to register multiple
pinned contexts to an engine? 

For the migration code itself, I figured reserving one copy engine for
migration was strictly needed for recoverable page-faults? In the
current version we're not doing that, but just tying a pinned migration
context to the first available copy engine on the gt, to be used when
we don't have a ww context available to pin a separate context using a
random copy engine. Note also the ring size of the migration contexts;
since we're populating the page-tables for each blit, it's not hard to
fill the ring and in the end multiple contexts I guess boils down to
avoiding priority inversion on migration, including blocking high
priority kernel context tasks.

As for not using the gt->vm, I'm not completely sure if we can do our
special page-table setup on that, Got to defer that question to Chris,
but once Ram's work of supporting 64K LMEM PTEs on that has landed I
guess we could easily reuse the gt->vm if possible and suitable.

Thanks,
/Thomas