[Intel-gfx] [PATCH 2/5] drm/i915: Introduce i915_timeline.mutex

[Tvrtko Ursulin]

On 01/03/2019 11:05, Chris Wilson wrote:
> A simple mutex used for guarding the flow of requests in and out of the
> timeline. In the short-term, it will be used only to guard the addition
> of requests into the timeline, taken on alloc and released on commit so
> that only one caller can construct a request into the timeline
> (important as the seqno and ring pointers must be serialised). This will
> be used by observers to ensure that the seqno/hwsp is stable. Later,
> when we have reduced retiring to only operate on a single timeline at a
> time, we can then use the mutex as the sole guard required for retiring.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/i915_request.c            | 6 +++++-
>   drivers/gpu/drm/i915/i915_timeline.c           | 1 +
>   drivers/gpu/drm/i915/i915_timeline.h           | 2 ++
>   drivers/gpu/drm/i915/selftests/i915_request.c  | 4 +---
>   drivers/gpu/drm/i915/selftests/mock_timeline.c | 1 +
>   5 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index c65f6c990fdd..719d1a5ab082 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -563,6 +563,7 @@ i915_request_alloc(struct intel_engine_cs *engine, struct i915_gem_context *ctx)
>   		return ERR_CAST(ce);
>   
>   	reserve_gt(i915);
> +	mutex_lock(&ce->ring->timeline->mutex);
>   
>   	/* Move our oldest request to the slab-cache (if not in use!) */
>   	rq = list_first_entry(&ce->ring->request_list, typeof(*rq), ring_link);
> @@ -688,6 +689,7 @@ i915_request_alloc(struct intel_engine_cs *engine, struct i915_gem_context *ctx)
>   
>   	kmem_cache_free(global.slab_requests, rq);
>   err_unreserve:
> +	mutex_unlock(&ce->ring->timeline->mutex);
>   	unreserve_gt(i915);
>   	intel_context_unpin(ce);
>   	return ERR_PTR(ret);
> @@ -880,7 +882,7 @@ void i915_request_add(struct i915_request *request)
>   	GEM_TRACE("%s fence %llx:%lld\n",
>   		  engine->name, request->fence.context, request->fence.seqno);
>   
> -	lockdep_assert_held(&request->i915->drm.struct_mutex);
> +	lockdep_assert_held(&request->timeline->mutex);
>   	trace_i915_request_add(request);
>   
>   	/*
> @@ -991,6 +993,8 @@ void i915_request_add(struct i915_request *request)
>   	 */
>   	if (prev && i915_request_completed(prev))
>   		i915_request_retire_upto(prev);
> +
> +	mutex_unlock(&request->timeline->mutex);
>   }
>   
>   static unsigned long local_clock_us(unsigned int *cpu)
> diff --git a/drivers/gpu/drm/i915/i915_timeline.c b/drivers/gpu/drm/i915/i915_timeline.c
> index b2202d2e58a2..87a80558da28 100644
> --- a/drivers/gpu/drm/i915/i915_timeline.c
> +++ b/drivers/gpu/drm/i915/i915_timeline.c
> @@ -162,6 +162,7 @@ int i915_timeline_init(struct drm_i915_private *i915,
>   	timeline->fence_context = dma_fence_context_alloc(1);
>   
>   	spin_lock_init(&timeline->lock);
> +	mutex_init(&timeline->mutex);
>   
>   	INIT_ACTIVE_REQUEST(&timeline->barrier);
>   	INIT_ACTIVE_REQUEST(&timeline->last_request);
> diff --git a/drivers/gpu/drm/i915/i915_timeline.h b/drivers/gpu/drm/i915/i915_timeline.h
> index 7bec7d2e45bf..36c3849f7108 100644
> --- a/drivers/gpu/drm/i915/i915_timeline.h
> +++ b/drivers/gpu/drm/i915/i915_timeline.h
> @@ -44,6 +44,8 @@ struct i915_timeline {
>   #define TIMELINE_CLIENT 0 /* default subclass */
>   #define TIMELINE_ENGINE 1
>   
> +	struct mutex mutex; /* protects the flow of requests */
> +
>   	unsigned int pin_count;
>   	const u32 *hwsp_seqno;
>   	struct i915_vma *hwsp_ggtt;
> diff --git a/drivers/gpu/drm/i915/selftests/i915_request.c b/drivers/gpu/drm/i915/selftests/i915_request.c
> index 7da52e3d67af..7e1b65b8eb19 100644
> --- a/drivers/gpu/drm/i915/selftests/i915_request.c
> +++ b/drivers/gpu/drm/i915/selftests/i915_request.c
> @@ -141,14 +141,12 @@ static int igt_fence_wait(void *arg)
>   		err = -ENOMEM;
>   		goto out_locked;
>   	}
> -	mutex_unlock(&i915->drm.struct_mutex); /* safe as we are single user */
>   
>   	if (dma_fence_wait_timeout(&request->fence, false, T) != -ETIME) {
>   		pr_err("fence wait success before submit (expected timeout)!\n");
> -		goto out_device;
> +		goto out_locked;
>   	}
>   
> -	mutex_lock(&i915->drm.struct_mutex);
>   	i915_request_add(request);
>   	mutex_unlock(&i915->drm.struct_mutex);
>   
> diff --git a/drivers/gpu/drm/i915/selftests/mock_timeline.c b/drivers/gpu/drm/i915/selftests/mock_timeline.c
> index d2de9ece2118..416d85233263 100644
> --- a/drivers/gpu/drm/i915/selftests/mock_timeline.c
> +++ b/drivers/gpu/drm/i915/selftests/mock_timeline.c
> @@ -14,6 +14,7 @@ void mock_timeline_init(struct i915_timeline *timeline, u64 context)
>   	timeline->fence_context = context;
>   
>   	spin_lock_init(&timeline->lock);
> +	mutex_init(&timeline->mutex);
>   
>   	INIT_ACTIVE_REQUEST(&timeline->barrier);
>   	INIT_ACTIVE_REQUEST(&timeline->last_request);
> 

So hard to predict if this will be okay in the future and keep a mental 
image of the endgame in ones head.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

Regards,

Tvrtko