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

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 11/06/2018 09:32, Chris Wilson wrote:
> 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@xxxxxxxxxxxxxxxxxx>
> > > Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> > > Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
> > > Cc: Matthew Auld <matthew.william.auld@xxxxxxxxx>
> > > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > > ---
> > >    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...

Silly me. Can you just make use of intel_ring_offset_valid from 
assert_ring_tail_valid so we don't duplicate the checks?

Regards,

Tvrtko
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