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

[Ben Widawsky <ben@bwidawsk.net>]

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@whitecape.org>
> Cc: Jesse Barnes <jbarnes@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);
> +
>  	/* Improve HiZ throughput on CHV. */
>  	WA_SET_BIT_MASKED(HIZ_CHICKEN, CHV_HZ_8X8_MODE_IN_1X);
>  

I think you should do this as two separate patches, 1 per platform. For the BSW
patch (given that I had the same functionality in the kernel patch I asked you
to look at ;-) and FWIW, Jordan has numbers on BSW B-step with my kernel patch
which we can use for the commit):
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>

I haven't looked at Broadwell docs, so I'll let someone else take care of that.

I don't know if I agree with Chris that we should call these in the workaround
section, but whatever. init_clock_gating is equally sucky.
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