[Mesa-dev] [Intel-gfx] [PATCH] drm/i915: Enable the HiZ RAW Stall Optimization on Gen8.

[Ville Syrjälä]

On Sat, Jan 10, 2015 at 06:44:49PM -0800, Kenneth Graunke wrote:
> This is an important optimization for avoiding read-after-write (RAW)
> stalls in the HiZ buffer.  Certain workloads would run very slowly with
> HiZ enabled, but run much faster with the "hiz=false" driconf option.
> With this patch, they run at full speed even with HiZ.
> 
> Improves performance in OglVSInstancing by 3.2x on Broadwell GT3e
> (Iris Pro 6200).
> 
> Thanks to Jesse Barnes for finding this missing bit!
> Thanks to Chris Wilson for helping me find where to set it.
> 
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> Cc: Jesse Barnes <jbarnes at virtuousgeek.org>
> ---
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> Here's an alternate patch which implements the workaround in the kernel
> instead of Mesa.  It's probably better to do it there, since the kernel
> does it on Haswell already.
> 
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index dabc1d8..23020d6 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -796,6 +796,16 @@ static int bdw_init_workarounds(struct intel_engine_cs *ring)
>  			  HDC_DONOT_FETCH_MEM_WHEN_MASKED |
>  			  (IS_BDW_GT3(dev) ? HDC_FENCE_DEST_SLM_DISABLE : 0));
>  
> +	/* From the Haswell PRM, Command Reference: Registers, CACHE_MODE_0:
> +	 * "The Hierarchical Z RAW Stall Optimization allows non-overlapping
> +	 *  polygons in the same 8x4 pixel/sample area to be processed without
> +	 *  stalling waiting for the earlier ones to write to Hierarchical Z
> +	 *  buffer."
> +	 *
> +	 * This optimization is off by default for Broadwell; turn it on.
> +	 */
> +	WA_CLR_BIT_MASKED(CACHE_MODE_0_GEN7, HIZ_RAW_STALL_OPT_DISABLE);
> +
>  	/* Wa4x4STCOptimizationDisable:bdw */
>  	WA_SET_BIT_MASKED(CACHE_MODE_1,
>  			  GEN8_4x4_STC_OPTIMIZATION_DISABLE);
> @@ -836,6 +846,11 @@ static int chv_init_workarounds(struct intel_engine_cs *ring)
>  			  HDC_FORCE_NON_COHERENT |
>  			  HDC_DONOT_FETCH_MEM_WHEN_MASKED);
>  
> +	/* According to the CACHE_MODE_0 default value documentation, some
> +	 * CHV platforms disable this optimization by default.  Turn it on.
> +	 */
> +	WA_CLR_BIT_MASKED(CACHE_MODE_0_GEN7, HIZ_RAW_STALL_OPT_DISABLE);
> +

I remember looking at this when the HSW version was done, and at the
time the docs claimed that the default value on gen8 was already good.
Looks like the docs have been updated since then. Also my BSW agrees
that the disable bit is now set by default.

I suppose we could assume that since it works on HSW, it'll work on
gen8. However, I find it a bit suspicious that the later steppings seem
to have changed the default to disable the optimization. IVB suffered
from real problems with the optimization enabled and hence the IVB patch
was reverted. IIRC Chia-I wrote some kind of test to demonstrate the
problem on IVB, so maybe you want to run it and make sure it still
works correctly on gen8. Here's the test:
http://permalink.gmane.org/gmane.comp.freedesktop.xorg.drivers.intel/35399

>  	/* Improve HiZ throughput on CHV. */
>  	WA_SET_BIT_MASKED(HIZ_CHICKEN, CHV_HZ_8X8_MODE_IN_1X);
>  
> -- 
> 2.2.1
> 
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-- 
Ville Syrjälä
Intel OTC