Re: [PATCH v4] drm/i915: Adjust size of PIPE_CONTROL used for gen8 render seqno write

[Abdiel Janulgue <abdiel.janulgue@linux.intel.com>]

On 04/12/2016 04:51 PM, Michał Winiarski wrote:
> We started to use PIPE_CONTROL to write render ring seqno in order to
> combat seqno write vs interrupt generation problems. This was introduced
> by commit 7c17d377374d ("drm/i915: Use ordered seqno write interrupt
> generation on gen8+ execlists").
> 
> On gen8+ size of PIPE_CONTROL with Post Sync Operation should be
> 6 dwords. When we're using older 5-dword variant it's possible to
> observe inconsistent values written by PIPE_CONTROL with Post
> Sync Operation from user batches, resulting in rendering corruptions.
> 
> v2: Fix BAT failures
> v3: Comments on alignment and thrashing high dword of seqno (Chris)
> v4: Updated commit msg (Mika)
> 
> Testcase: igt/gem_pipe_control_store_loop/*-qword-write
> Issue: VIZ-7393
> Cc: stable@vger.kernel.org
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@intel.com>
> Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>

Tested-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>


> Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_lrc.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 0d6dc5e..30abe53 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -1945,15 +1945,18 @@ static int gen8_emit_request_render(struct drm_i915_gem_request *request)
>  	struct intel_ringbuffer *ringbuf = request->ringbuf;
>  	int ret;
>  
> -	ret = intel_logical_ring_begin(request, 6 + WA_TAIL_DWORDS);
> +	ret = intel_logical_ring_begin(request, 8 + WA_TAIL_DWORDS);
>  	if (ret)
>  		return ret;
>  
> +	/* We're using qword write, seqno should be aligned to 8 bytes. */
> +	BUILD_BUG_ON(I915_GEM_HWS_INDEX & 1);
> +
>  	/* w/a for post sync ops following a GPGPU operation we
>  	 * need a prior CS_STALL, which is emitted by the flush
>  	 * following the batch.
>  	 */
> -	intel_logical_ring_emit(ringbuf, GFX_OP_PIPE_CONTROL(5));
> +	intel_logical_ring_emit(ringbuf, GFX_OP_PIPE_CONTROL(6));
>  	intel_logical_ring_emit(ringbuf,
>  				(PIPE_CONTROL_GLOBAL_GTT_IVB |
>  				 PIPE_CONTROL_CS_STALL |
> @@ -1961,7 +1964,10 @@ static int gen8_emit_request_render(struct drm_i915_gem_request *request)
>  	intel_logical_ring_emit(ringbuf, hws_seqno_address(request->engine));
>  	intel_logical_ring_emit(ringbuf, 0);
>  	intel_logical_ring_emit(ringbuf, i915_gem_request_get_seqno(request));
> +	/* We're thrashing one dword of HWS. */
> +	intel_logical_ring_emit(ringbuf, 0);
>  	intel_logical_ring_emit(ringbuf, MI_USER_INTERRUPT);
> +	intel_logical_ring_emit(ringbuf, MI_NOOP);
>  	return intel_logical_ring_advance_and_submit(request);
>  }
>  
> 
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