[Intel-gfx] [PATCH 13/73] drm/i915: Move the modulus for ring emission to the register write

[Joonas Lahtinen]

On ma, 2016-08-01 at 11:15 +0100, Chris Wilson wrote:
> On Mon, Aug 01, 2016 at 01:07:55PM +0300, Joonas Lahtinen wrote:
> > 
> > On ma, 2016-08-01 at 10:10 +0100, Chris Wilson wrote:
> > > 
> > > Space reservation is already safe with respect to the ring->size
> > > modulus, but hardware only expects to see values in the range
> > > 0...ring->size-1 (inclusive) and so requires the modulus to prevent us
> > > writing the value ring->size instead of 0. As this is only required for
> > > the register itself, we can defer the modulus to the register update and
> > > not perform it after every command packet. We keep the
> > > intel_ring_advance() around in the code to provide demarcation for the
> > > end-of-packet (which then can be compared against intel_ring_begin() as
> > > the number of dwords emitted must match the reserved space).
> > > 
> > > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > > Cc: Dave Gordon <david.s.gordon at intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/intel_lrc.c        |  2 +-
> > >  drivers/gpu/drm/i915/intel_ringbuffer.c |  6 ++++--
> > >  drivers/gpu/drm/i915/intel_ringbuffer.h | 17 +++++++++++++----
> > >  3 files changed, 18 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> > > index bf42a66d6624..824f7efe4e64 100644
> > > --- a/drivers/gpu/drm/i915/intel_lrc.c
> > > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> > > @@ -373,7 +373,7 @@ static void execlists_update_context(struct drm_i915_gem_request *rq)
> > >  	struct i915_hw_ppgtt *ppgtt = rq->ctx->ppgtt;
> > >  	uint32_t *reg_state = rq->ctx->engine[engine->id].lrc_reg_state;
> > >  
> > > -	reg_state[CTX_RING_TAIL+1] = rq->tail;
> > > +	reg_state[CTX_RING_TAIL+1] = intel_ring_offset(rq->ring, rq->tail);
> > >  
> > >  	/* True 32b PPGTT with dynamic page allocation: update PDP
> > >  	 * registers and point the unallocated PDPs to scratch page.
> > > diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> > > index 3142085b5cc0..21d5e8209400 100644
> > > --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> > > +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> > > @@ -1718,7 +1718,8 @@ static void i9xx_submit_request(struct drm_i915_gem_request *request)
> > >  {
> > >  	struct drm_i915_private *dev_priv = request->i915;
> > >  
> > > -	I915_WRITE_TAIL(request->engine, request->tail);
> > > +	I915_WRITE_TAIL(request->engine,
> > > +			intel_ring_offset(request->ring, request->tail));
> > >  }
> > >  
> > >  static void
> > > @@ -2505,7 +2506,8 @@ static void gen6_bsd_submit_request(struct drm_i915_gem_request *request)
> > >  		DRM_ERROR("timed out waiting for the BSD ring to wake up\n");
> > >  
> > >  	/* Now that the ring is fully powered up, update the tail */
> > > -	I915_WRITE_FW(RING_TAIL(request->engine->mmio_base), request->tail);
> > > +	I915_WRITE_FW(RING_TAIL(request->engine->mmio_base),
> > > +		      intel_ring_offset(request->ring, request->tail));
> > >  	POSTING_READ_FW(RING_TAIL(request->engine->mmio_base));
> > >  
> > >  	/* Let the ring send IDLE messages to the GT again,
> > > diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
> > > index 14d2ea36fb88..9ac96ddb01ee 100644
> > > --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> > > +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> > > @@ -460,14 +460,23 @@ static inline void intel_ring_emit_reg(struct intel_ring *ring, i915_reg_t reg)
> > >  
> > >  static inline void intel_ring_advance(struct intel_ring *ring)
> > >  {
> > > +	/* Dummy function.
> > > +	 *
> > > +	 * This serves as a placeholder in the code so that the reader
> > > +	 * can compare against the preceding intel_ring_begin() and
> > > +	 * check that the number of dwords emitted matches the space
> > > +	 * reserved for the command packet (i.e. the value passed to
> > > +	 * intel_ring_begin()).
> > > +	 */
> > > +}
> > > +
> > > +static inline u32 intel_ring_offset(struct intel_ring *ring, u32 value)
> > > +{
> > >  	/* The modulus is required so that we avoid writing
> > >  	 * request->tail == ring->size, rather than the expected 0,
> > >  	 * into the RING_TAIL register as that can cause a GPU hang.
> > > -	 * As this is only strictly required for the request->tail,
> > > -	 * and only then as we write the value into hardware, we can
> > > -	 * one day remove the modulus after every command packet.
> > >  	 */
> > > -	ring->tail &= ring->size - 1;
> > > +	return value & (ring->size - 1);
> > >  }
> > The comment seems outdated-ish as it speaks of modulus which is nowhere
> > to be seen. I'd speak of 'masking'. With that,
> The operation is a modulus. The common optimisation for moduli with
> a power-of-two divisor being applied.

Yes, I'm perfectly aware of that, but as discussed in IRC, that
information would be good to be local. Modulus in this case a trick to
achieve value == ring->size ? 0 : value; Which would again be more
informative, we do not expect wrap, but we have one special value.

Regards, Joonas

> -Chris
> 
-- 
Joonas Lahtinen
Open Source Technology Center
Intel Corporation