[Intel-gfx] [PATCH v4 2/3] drm/i915: Make sure engines are idle during GPU idling in LR mode

[Chris Wilson]

On Sat, Nov 05, 2016 at 10:36:00AM +0200, Imre Deak wrote:
> We assume that the GPU is idle once receiving the seqno via the last
> request's user interrupt. In execlist mode the corresponding context
> completed interrupt can be delayed though and until this latter
> interrupt arrives we consider the request to be pending on the ELSP
> submit port. This can cause a problem during system suspend where this
> last request will be seen by the resume code as still pending. Such
> pending requests are normally replayed after a GPU reset, but during
> resume we reset both SW and HW tracking of the ring head/tail pointers,
> so replaying the pending request with its stale tail pointer will leave
> the ring in an inconsistent state. A subsequent request submission can
> lead then to the GPU executing from uninitialized area in the ring
> behind the above stale tail pointer.
> 
> Fix this by making sure any pending request on the ELSP port is
> completed before suspending. I used a polling wait since the completion
> time I measured was <1ms and since normally we only need to wait during
> system suspend. GPU idling during runtime suspend is scheduled with a
> delay (currently 50-100ms) after the retirement of the last request at
> which point the context completed interrupt must have arrived already.
> 
> The chance of this bug was increased by
> 
> commit 1c777c5d1dcdf8fa0223fcff35fb387b5bb9517a
> Author: Imre Deak <imre.deak at intel.com>
> Date:   Wed Oct 12 17:46:37 2016 +0300
> 
>     drm/i915/hsw: Fix GPU hang during resume from S3-devices state
> 
> but it could happen even without the explicit GPU reset, since we
> disable interrupts afterwards during the suspend sequence.
> 
> v2:
> - Do an unlocked poll-wait first. (Chris)
> v3-4:
> - s/intel_lr_engines_idle/intel_execlists_idle/ and move
>   i915.enable_execlists check to the new helper. (Chris)
> 
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala at intel.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98470
> Signed-off-by: Imre Deak <imre.deak at intel.com>
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre