[Intel-gfx] [PATCH 3/6] drm/i915: Update ring position from request on retiring

[Chris Wilson]

Quoting Mika Kuoppala (2018-03-09 13:38:37)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > When wedged, we do not update the ring->tail as we submit the requests
> > causing us to leak the ring->space upon cleaning up the wedged driver.
> > We can just use the value stored in rq->tail, and keep the submission
> > backend details away from set-wedge.
> >
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/i915_request.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> > index efa9ee557f31..69b378a323fc 100644
> > --- a/drivers/gpu/drm/i915/i915_request.c
> > +++ b/drivers/gpu/drm/i915/i915_request.c
> > @@ -358,7 +358,7 @@ static void advance_ring(struct i915_request *request)
> >                * is just about to be. Either works, if we miss the last two
> >                * noops - they are safe to be replayed on a reset.
> >                */
> > -             tail = READ_ONCE(request->ring->tail);
> > +             tail = READ_ONCE(request->tail);
> 
> I tried to think if we need the READ_ONCE here anymore.

I tried as well to see if the comment was still correct. It still is due
to that we can retire before we see the context-switch interrupt.
 
> But as this is the safest version,
> Reviewed-by: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> 
> Noticed that request->tail is not cleared on i915_request_alloc.
> 
> If we would set rq->head = rq->tail = rq->ring->emit
> we could use rq->head == rq->tail to assert that
> we screw up something major during the request lifetime.

Heh, if we get a stray write here, we're going to get stray writes
everywhere :)
-Chris