[Intel-gfx] [PATCH v3] drm/i915/execlists: Reset CSB write pointer after reset

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-06-28 13:15:07)
> 
> On 28/06/2018 12:59, Chris Wilson wrote:
> > On HW reset, the HW clears the write pointer (to 0). But since it also
> > writes its first CSB entry to slot 0, we need to reset the write pointer
> > back to the element before (so the first entry we read is 0).
> > 
> > This is required for the next patch, where we trust the CSB completely!
> > 
> > v2: Use _MASKED_FIELD
> > v3: Store the reset value, so that we differentiate between mmio/hwsp
> > transparently and without pretense.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > ---
> >   drivers/gpu/drm/i915/intel_lrc.c        | 23 +++++++++++++++++++++--
> >   drivers/gpu/drm/i915/intel_ringbuffer.h |  9 +++++++++
> >   2 files changed, 30 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> > index 8531a5b6f6ff..49bf5048043c 100644
> > --- a/drivers/gpu/drm/i915/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> > @@ -884,6 +884,21 @@ static void reset_irq(struct intel_engine_cs *engine)
> >       clear_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted);
> >   }
> >   
> > +static void reset_csb_pointers(struct intel_engine_execlists *execlists)
> > +{
> > +     /*
> > +      * After a reset, the HW starts writing into CSB entry [0]. We
> > +      * therefore have to set our HEAD pointer back one entry so that
> > +      * the *first* entry we check is entry 0. To complicate this further,
> > +      * as we don't wait for the first interrupt after reset, we have to
> > +      * fake the HW write to point back to the last entry so that our
> > +      * inline comparison of our cached head position against the last HW
> > +      * write works even before the first interrupt.
> > +      */
> > +     execlists->csb_head = execlists->csb_write_reset & 0xff;
> 
> Idea for avoiding the & 0xff and applying the trick throughout - make 
> csb_head u8. :)

Just leaves intel_engine_execlists full of holes. It did close my mind.
They may just spontaneously rearrange themselves...
-Chris