[Intel-gfx] [PATCH 2/3] drm/i915/guc: Keep the previous context pinned until the next one has been completed

[Chris Wilson]

On Fri, Apr 15, 2016 at 12:54:34PM +0100, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> In GuC mode submitting requests is only putting them into the
> GuC queue with the actual submission to hardware following at
> some future point. This makes the per engine last context
> tracking insufficient for closing the premature context
> unpin race.
> 
> Instead we need to make requests track and pin the previous
> context on the same engine, and only unpin them when they
> themselves are retired. This will ensure contexts remain pinned
> in all cases until the are fully saved by the GPU.
> 
> v2: Only track contexts.
> v3: Limit to GuC for now.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Alex Dai <yu.dai at intel.com>
> Cc: Dave Gordon <david.s.gordon at intel.com>
> Cc: Nick Hoath <nicholas.hoath at intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/i915_drv.h  |  4 +++-
>  drivers/gpu/drm/i915/i915_gem.c  |  5 +++++
>  drivers/gpu/drm/i915/intel_lrc.c | 15 +++++++++++++++
>  3 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index b9ed1b305beb..80f2d959ed7b 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2290,6 +2290,9 @@ struct drm_i915_gem_request {
>  	struct intel_context *ctx;
>  	struct intel_ringbuffer *ringbuf;
>  
> +	/** Context preceding this one on the same engine. Can be NULL. */
> +	struct intel_context *prev_ctx;
> +
>  	/** Batch buffer related to this request if any (used for
>  	    error state dump only) */
>  	struct drm_i915_gem_object *batch_obj;
> @@ -2325,7 +2328,6 @@ struct drm_i915_gem_request {
>  
>  	/** Execlists no. of times this request has been sent to the ELSP */
>  	int elsp_submitted;
> -
>  };
>  
>  struct drm_i915_gem_request * __must_check
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index e9db56c7a31f..18e747132ce5 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1413,6 +1413,11 @@ static void i915_gem_request_retire(struct drm_i915_gem_request *request)
>  	list_del_init(&request->list);
>  	i915_gem_request_remove_from_client(request);
>  
> +	if (request->prev_ctx) {
> +		intel_lr_context_unpin(request->prev_ctx, request->engine);
> +		request->prev_ctx = NULL;
> +	}
> +
>  	i915_gem_request_unreference(request);
>  }
>  
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 2f49dfae5560..73be9eac8ecc 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -791,6 +791,21 @@ intel_logical_ring_advance_and_submit(struct drm_i915_gem_request *request)
>  	if (intel_engine_stopped(engine))
>  		return 0;
>  
> +	/*
> +	 * Pin the previous context on this engine to ensure it is not
> +	 * prematurely unpinned in GuC mode.
> +	 * Previous context will be unpinned when this request is retired,
> +	 * ensuring the GPU has switched out from the previous context and
> +	 * into a new context at that point.
> +	 */
> +	if (dev_priv->guc.execbuf_client && engine->last_context) {
> +		intel_lr_context_pin(engine->last_context, engine);
> +		request->prev_ctx = engine->last_context;
> +	}

Nak.

We just want,

request->pinned_ctx = engine->last_context;
engine->last_context = request->ctx;

as that works for everyone (and replaces the current last_context dance)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre