[Intel-gfx] [PATCH] drm/i915: Wrap around the tail offset before setting ring->tail

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-06-11 09:28:22)
> 
> On 08/06/2018 18:25, Chris Wilson wrote:
> > The HW only accepts offsets within ring->size, and fails peculiarly if
> > the RING_HEAD or RING_TAIL is set to ring->size. Therefore whenever we
> > set ring->head/ring->tail we want to make sure it is within value (using
> > intel_ring_wrap()).
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> > Cc: Matthew Auld <matthew.william.auld at gmail.com>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > ---
> >   drivers/gpu/drm/i915/intel_ringbuffer.c |  5 +++++
> >   drivers/gpu/drm/i915/intel_ringbuffer.h | 12 ++++++++++++
> >   2 files changed, 17 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> > index 6ac3b65373fe..9fac0e0f078e 100644
> > --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> > +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> > @@ -496,6 +496,10 @@ static int init_ring_common(struct intel_engine_cs *engine)
> >               DRM_DEBUG_DRIVER("%s initialization failed [head=%08x], fudging\n",
> >                                engine->name, I915_READ_HEAD(engine));
> >   
> > +     /* Check that the ring offsets point within the ring! */
> > +     GEM_BUG_ON(!intel_ring_offset_valid(ring, ring->head));
> > +     GEM_BUG_ON(!intel_ring_offset_valid(ring, ring->tail));
> > +
> >       intel_ring_update_space(ring);
> >       I915_WRITE_HEAD(engine, ring->head);
> >       I915_WRITE_TAIL(engine, ring->tail);
> > @@ -1064,6 +1068,7 @@ int intel_ring_pin(struct intel_ring *ring,
> >   
> >   void intel_ring_reset(struct intel_ring *ring, u32 tail)
> >   {
> > +     tail = intel_ring_wrap(ring, tail);
> >       ring->tail = tail;
> >       ring->head = tail;
> >       ring->emit = tail;
> > diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
> > index b44c67849749..1d8140ac2016 100644
> > --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> > +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> > @@ -805,6 +805,18 @@ static inline u32 intel_ring_wrap(const struct intel_ring *ring, u32 pos)
> >       return pos & (ring->size - 1);
> >   }
> >   
> > +static inline bool
> > +intel_ring_offset_valid(const struct intel_ring *ring, u32 pos)
> > +{
> > +     if (pos & -ring->size) /* must be strictly within the ring */
> > +             return false;
> > +
> > +     if (!IS_ALIGNED(pos, 8)) /* must be qword aligned */
> > +             return false;
> > +
> > +     return true;
> > +}
> 
> Looks like we have assert_ring_tail_valid and intel_ring_set_tail 
> already in the tree. So could just use the latter in intel_ring_reset if 
> needed.

No, because that is only setting the tail. The problem also lies in that
we set RING_HEAD and so need to be careful about the value we assign to
ring->head.

> But also since intel_ring_reset is only setting the tail to 
> either zero or existing ring->tail it sounds like the check would be 
> better placed where the tail advances?

Which we do. The problem is not the tail...
-Chris