Re: [PATCH] drm/i915: Reset the HEAD pointer for the ring after writing START

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Aug 07, 2014 at 03:39:54PM +0100, Chris Wilson wrote:
> Ville found an old w/a documented for g4x that suggested that we need to
> reset the HEAD after writing START. This is a useful fixup for some of
> the g4x ring initialisation woes, but as usual, not all.
> 
> v2: Do the rewrite unconditionally anyway
> 
> References: https://bugs.freedesktop.org/show_bug.cgi?id=76554
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>

Yeah looks sensible to me. So based on my gut feeling just from the
seeing the w/a name this is:

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> ---
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index 45e3ec927051..26ec25afc02a 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -537,6 +537,14 @@ static int init_ring_common(struct intel_engine_cs *ring)
>  	 * also enforces ordering), otherwise the hw might lose the new ring
>  	 * register values. */
>  	I915_WRITE_START(ring, i915_gem_obj_ggtt_offset(obj));
> +
> +	/* WaClearRingBufHeadRegAtInit:ctg,elk */
> +	if (I915_READ_HEAD(ring))
> +		DRM_DEBUG("%s initialization failed [head=%08x], fudging\n",
> +			  ring->name, I915_READ_HEAD(ring));
> +	I915_WRITE_HEAD(ring, 0);
> +	(void)I915_READ_HEAD(ring);
> +
>  	I915_WRITE_CTL(ring,
>  			((ringbuf->size - PAGE_SIZE) & RING_NR_PAGES)
>  			| RING_VALID);
> -- 
> 2.1.0.rc1

-- 
Ville Syrjälä
Intel OTC