[Intel-gfx] [PATCH 1/3] drm/i915: Only spin whilst waiting on the current request

[Chris Wilson]

Limit busywaiting only to the request currently being processed by the
GPU. If the request is not currently being processed by the GPU, there
is a very low likelihood of it being completed within the 2 microsecond
spin timeout and so we will just be wasting CPU cycles.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_drv.h | 2 +-
 drivers/gpu/drm/i915/i915_gem.c | 8 +++++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 8afda459a26e..16095b95d2df 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2190,7 +2190,7 @@ struct drm_i915_gem_request {
 	struct intel_engine_cs *ring;
 
 	/** GEM sequence number associated with this request. */
-	uint32_t seqno;
+	uint32_t seqno, spin_seqno;
 
 	/** Position in the ringbuffer of the start of the request */
 	u32 head;
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 414150a0b8d5..af9ffa11ef44 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1193,9 +1193,14 @@ static int __i915_spin_request(struct drm_i915_gem_request *req, int state)
 	 * takes to sleep on a request, on the order of a microsecond.
 	 */
 
-	if (i915_gem_request_get_ring(req)->irq_refcount)
+	if (req->ring->irq_refcount)
 		return -EBUSY;
 
+	/* Only spin if we know the GPU is processing this request */
+	if (i915_seqno_passed(req->ring->get_seqno(req->ring, false),
+			      req->spin_seqno))
+		return -EAGAIN;
+
 	timeout = local_clock_us(&cpu) + 2;
 	while (!need_resched()) {
 		if (i915_gem_request_completed(req, true))
@@ -2592,6 +2597,7 @@ void __i915_add_request(struct drm_i915_gem_request *request,
 	request->batch_obj = obj;
 
 	request->emitted_jiffies = jiffies;
+	request->spin_seqno = ring->last_submitted_seqno;
 	ring->last_submitted_seqno = request->seqno;
 	list_add_tail(&request->list, &ring->request_list);
 
-- 
2.6.2