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@xxxxxxxxxxxxxxxxxx>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > ---
> > 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
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