[Intel-gfx] [RFC 2/4] drm/i915/guc: Keep the previous context pinned until the next one has been completed

[Chris Wilson]

On Fri, Apr 08, 2016 at 02:54:56PM +0100, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko at ursulin.net>
> 
> 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.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko at ursulin.net>
> 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 | 17 +++++++++++++++++
>  3 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index fa199b7e51b5..216b2297ae3e 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2283,6 +2283,9 @@ 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;
> @@ -2318,7 +2321,6 @@ 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 c1df696f9002..118911ce7231 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1445,6 +1445,11 @@ static void i915_gem_request_retire(struct drm_i915_gem_request *request)
>  	 */
>  	request->ringbuf->last_retired_head = request->postfix;
>  
> +	if (request->prev_ctx) {
> +		intel_lr_context_unpin(request->prev_ctx, request->engine);
> +		request->prev_ctx = NULL;
> +	}
> +
>  	__i915_gem_request_release(request);
>  }
>  
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 31445aa3429b..3d08dac0a9b0 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -798,6 +798,23 @@ 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 (engine->last_context) {
> +		if (engine->last_context != request->i915->kernel_context) {
> +			intel_lr_context_pin(engine->last_context, engine);
> +			request->prev_ctx = engine->last_context;
> +		}
> +	}
> +
> +	/*
> +	 * Track and pin the last context submitted on an engine.
> +	 */
>  	if (engine->last_context != request->ctx) {
>  		if (engine->last_context)
>  			intel_lr_context_unpin(engine->last_context, engine);

We can use pinned_context to reduce the code complexity, not make it
worse.

If you first apply the execlists default context fix, adding
pinned_context is a reduction is code.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre