Re: [PATCH v3] drm/i915/guc: Move GuC wq_check_space to alloc_request_extras

[Dave Gordon <david.s.gordon@intel.com>]

On 11/12/15 01:17, yu.dai@intel.com wrote:
> From: Alex Dai <yu.dai@intel.com>
>
> Split GuC work queue space checking from submission and move it to
> ring_alloc_request_extras. The reason is that failure in later
> i915_add_request() won't be handled. In the case timeout happens,
> driver can return early in order to handle the error.
>
> v1: Move wq_reserve_space to ring_reserve_space
> v2: Move wq_reserve_space to alloc_request_extras (Chris Wilson)
> v3: The work queue head pointer is cached by driver now. So we can
>      quickly return if space is available.
>      s/reserve/check/g (Dave Gordon)
>
> Signed-off-by: Alex Dai <yu.dai@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_guc_submission.c | 34 ++++++++++++++++++------------
>   drivers/gpu/drm/i915/intel_guc.h           |  2 ++
>   drivers/gpu/drm/i915/intel_lrc.c           |  6 ++++++
>   3 files changed, 28 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_guc_submission.c b/drivers/gpu/drm/i915/i915_guc_submission.c
> index 226e9c0..cb8e1f71 100644
> --- a/drivers/gpu/drm/i915/i915_guc_submission.c
> +++ b/drivers/gpu/drm/i915/i915_guc_submission.c
> @@ -207,6 +207,9 @@ static int guc_ring_doorbell(struct i915_guc_client *gc)
>   	/* Update the tail so it is visible to GuC */
>   	desc->tail = gc->wq_tail;
>
> +	/* Cache the head where GuC is processing */
> +	gc->wq_head = desc->head;
> +

This could be at the end of this function, thus:

		/* update the cookie to newly read cookie from GuC */
		db_cmp.cookie = db_ret.cookie;
		db_exc.cookie = db_ret.cookie + 1;
		if (db_exc.cookie == 0)
			db_exc.cookie = 1;
	}

+	/* Finally, update the cached copy of the GuC's WQ head */
+	gc->wq_head = desc->head;
+
	kunmap_atomic(base);
	return ret;
}

which might increase the chance that we will get an updated value and 
thus avoid having to spinwait on the next check().

>   	/* current cookie */
>   	db_cmp.db_status = GUC_DOORBELL_ENABLED;
>   	db_cmp.cookie = gc->cookie;
> @@ -472,28 +475,30 @@ static void guc_fini_ctx_desc(struct intel_guc *guc,
>   			     sizeof(desc) * client->ctx_index);
>   }
>
> -/* Get valid workqueue item and return it back to offset */
> -static int guc_get_workqueue_space(struct i915_guc_client *gc, u32 *offset)
> +int i915_guc_wq_check_space(struct i915_guc_client *gc)
>   {
>   	struct guc_process_desc *desc;
>   	void *base;
>   	u32 size = sizeof(struct guc_wq_item);
>   	int ret = -ETIMEDOUT, timeout_counter = 200;
>
> +	if (!gc)
> +		return 0;
> +
> +	/* Quickly return if wq space is available since last time we cache the
> +	 * head position. */
> +	if (CIRC_SPACE(gc->wq_tail, gc->wq_head, gc->wq_size) >= size)
> +		return 0;
> +
>   	base = kmap_atomic(i915_gem_object_get_page(gc->client_obj, 0));
>   	desc = base + gc->proc_desc_offset;
>
>   	while (timeout_counter-- > 0) {
> -		if (CIRC_SPACE(gc->wq_tail, desc->head, gc->wq_size) >= size) {
> -			*offset = gc->wq_tail;
> +		gc->wq_head = desc->head;
>
> -			/* advance the tail for next workqueue item */
> -			gc->wq_tail += size;
> -			gc->wq_tail &= gc->wq_size - 1;
> -
> -			/* this will break the loop */
> -			timeout_counter = 0;
> +		if (CIRC_SPACE(gc->wq_tail, gc->wq_head, gc->wq_size) >= size) {
>   			ret = 0;
> +			break;
>   		}
>
>   		if (timeout_counter)
> @@ -512,11 +517,8 @@ static int guc_add_workqueue_item(struct i915_guc_client *gc,
>   	struct guc_wq_item *wqi;
>   	void *base;
>   	u32 tail, wq_len, wq_off = 0;
> -	int ret;
>
> -	ret = guc_get_workqueue_space(gc, &wq_off);
> -	if (ret)
> -		return ret;
> +	wq_off = gc->wq_tail;

Conversely, I'd prefer the updates below to be here instead; also the 
check that I mentioned last time:

+	u32 space = CIRC_SPACE(gc->wq_tail, gc->wq_head, gc->wq_size);
+	if (WARN_ON(space < sizeof(struct guc_wq_item))
+		return -ENOSPC;		/* shouldn't happen	*/
+
+	/* postincrement WQ tail for next time */
+	wq_off = gc->wq_tail;
+	gc->wq_tail += sizeof(struct guc_wq_item);
+	gc->wq_tail &= gc->wq_size - 1;

>
>   	/* For now workqueue item is 4 DWs; workqueue buffer is 2 pages. So we
>   	 * should not have the case where structure wqi is across page, neither
> @@ -551,6 +553,10 @@ static int guc_add_workqueue_item(struct i915_guc_client *gc,
>
>   	kunmap_atomic(base);
>
> +	/* advance the tail for next workqueue item */
> +	gc->wq_tail += sizeof(struct guc_wq_item);
> +	gc->wq_tail &= gc->wq_size - 1;

(moved to above)

> +
>   	return 0;
>   }
>
> diff --git a/drivers/gpu/drm/i915/intel_guc.h b/drivers/gpu/drm/i915/intel_guc.h
> index 0e048bf..5cf555d 100644
> --- a/drivers/gpu/drm/i915/intel_guc.h
> +++ b/drivers/gpu/drm/i915/intel_guc.h
> @@ -43,6 +43,7 @@ struct i915_guc_client {
>   	uint32_t wq_offset;
>   	uint32_t wq_size;
>   	uint32_t wq_tail;
> +	uint32_t wq_head;
>
>   	/* GuC submission statistics & status */
>   	uint64_t submissions[I915_NUM_RINGS];
> @@ -123,5 +124,6 @@ int i915_guc_submit(struct i915_guc_client *client,
>   		    struct drm_i915_gem_request *rq);
>   void i915_guc_submission_disable(struct drm_device *dev);
>   void i915_guc_submission_fini(struct drm_device *dev);
> +int i915_guc_wq_check_space(struct i915_guc_client *client);
>
>   #endif
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index f96fb51..9605ce9 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -667,6 +667,12 @@ int intel_logical_ring_alloc_request_extras(struct drm_i915_gem_request *request
>   			return ret;
>   	}
>
> +	/* Reserve GuC WQ space here (one request needs one WQ item) because
> +	 * the later i915_add_request() call can't fail. */
> +	ret = i915_guc_wq_check_space(request->i915->guc.execbuf_client);
> +	if (ret)
> +		return ret;
> +
>   	return 0;
>   }

It looks odd to call a GuC-only function regardless of whether we're 
using the GuC, so:

+	if (i915.enable_guc_submission) {
+		/*
+		 * Check that the GuC has space for the request before
+		 * going any further, as the i915_add_request() call
+		 * later on mustn't fail ...
+		 */
+		struct intel_guc *guc = &request->i915->guc;
+
+		ret = i915_guc_wq_check_space(guc->execbuf_client);
+		if (ret)
+			return ret;
+	}

.Dave.
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