Re: [PATCH 3/4] drm/i915: Reload PD tables after semaphore wait on gen8

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

On Sat, Apr 09, 2016 at 11:14:45AM +0100, Chris Wilson wrote:
> When the engine idles waiting upon a semaphore, it loses its
> pagetables and we must reload them before executing the batch.

RCS shouldn't lose them IIRC, other engines yes. As I recall the
other way to avoid this would be to prevent the engine from
entering some power save mode during the wait, but I suppose
that might come at a power cost.

> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index 62d09cf2ea8f..a4bab6466388 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -1446,6 +1446,7 @@ gen8_ring_sync(struct drm_i915_gem_request *waiter_req,
>  {
>  	struct intel_engine_cs *waiter = waiter_req->engine;
>  	struct drm_i915_private *dev_priv = waiter->dev->dev_private;
> +	struct i915_hw_ppgtt *ppgtt;
>  	int ret;
>  
>  	ret = intel_ring_begin(waiter_req, 4);
> @@ -1461,6 +1462,15 @@ gen8_ring_sync(struct drm_i915_gem_request *waiter_req,
>  	intel_ring_emit(waiter,
>  			upper_32_bits(GEN8_WAIT_OFFSET(waiter, signaller->id)));
>  	intel_ring_advance(waiter);
> +
> +	/* When the engine idles waiting upon a semaphore, it loses its
> +	 * pagetables and we must reload them before executing the batch.
> +	 * We do this on the i915_switch_context() following the wait and
> +	 * before the dispatch.
> +	 */
> +	ppgtt = waiter_req->ctx->ppgtt;
> +	if (ppgtt)
> +		ppgtt->pd_dirty_rings |= intel_engine_flag(waiter_req->engine);
>  	return 0;
>  }
>  
> -- 
> 2.8.0.rc3
> 
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-- 
Ville Syrjälä
Intel OTC
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