[Intel-gfx] [PATCH 21/32] drm/i915: Broadwell execlists needs exactly the same seqno w/a as legacy

[Jesse Barnes]

On 12/11/2015 03:33 AM, Chris Wilson wrote:
> +	 * Note that this effectively effectively stalls the read by the time
> +	 * it takes to do a memory transaction, which more or less ensures
> +	 * that the write from the GPU has sufficient time to invalidate
> +	 * the CPU cacheline. Alternatively we could delay the interrupt from
> +	 * the CS ring to give the write time to land, but that would incur
> +	 * a delay after every batch i.e. much more frequent than a delay
> +	 * when waiting for the interrupt (with the same net latency).
>  	 */
> +	struct drm_i915_private *dev_priv = ring->i915;
> +	POSTING_READ_FW(RING_ACTHD(ring->mmio_base));
> +
>  	intel_flush_status_page(ring, I915_GEM_HWS_INDEX);

Funnily enough, the interrupt ought to provide the same behavior as the MMIO read, i.e. flush outstanding system memory writes ahead of it.  The fact that we need it *plus* a CPU cache flush definitely means we're still missing something...

But hey, whatever works is good for now...

Jesse