[PATCH 0/2] drm/msm: Reduce fence signal latency

[PATCH 0/2] drm/msm: Reduce fence signal latency

[Rob Clark]

From: Rob Clark <robdclark@chromium.org>

A couple tweaks to reduce fence signal latency.

Rob Clark (2):
  drm/msm: Let fences read directly from memptrs
  drm/msm: Signal fences sooner

 drivers/gpu/drm/msm/msm_fence.c      | 11 +++++--
 drivers/gpu/drm/msm/msm_fence.h      | 41 +++++++++++++++++++++++---
 drivers/gpu/drm/msm/msm_gpu.c        | 44 ++++++++++++++++------------
 drivers/gpu/drm/msm/msm_ringbuffer.c |  2 +-
 4 files changed, 73 insertions(+), 25 deletions(-)

-- 
2.31.1

[PATCH 1/2] drm/msm: Let fences read directly from memptrs

[Rob Clark]

From: Rob Clark <robdclark@chromium.org>

Let dma_fence::signaled, etc, read directly from the address that the hw
is writing with updated completed fence seqno, so we can potentially
notice that the fence is signaled sooner.

Plus add some docs.

Signed-off-by: Rob Clark <robdclark@chromium.org>
---
 drivers/gpu/drm/msm/msm_fence.c      | 11 ++++++--
 drivers/gpu/drm/msm/msm_fence.h      | 41 +++++++++++++++++++++++++---
 drivers/gpu/drm/msm/msm_ringbuffer.c |  2 +-
 3 files changed, 47 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/msm/msm_fence.c b/drivers/gpu/drm/msm/msm_fence.c
index cd59a5918038..b92a9091a1e2 100644
--- a/drivers/gpu/drm/msm/msm_fence.c
+++ b/drivers/gpu/drm/msm/msm_fence.c
@@ -11,7 +11,8 @@
 
 
 struct msm_fence_context *
-msm_fence_context_alloc(struct drm_device *dev, const char *name)
+msm_fence_context_alloc(struct drm_device *dev, volatile uint32_t *fenceptr,
+		const char *name)
 {
 	struct msm_fence_context *fctx;
 
@@ -22,6 +23,7 @@ msm_fence_context_alloc(struct drm_device *dev, const char *name)
 	fctx->dev = dev;
 	strncpy(fctx->name, name, sizeof(fctx->name));
 	fctx->context = dma_fence_context_alloc(1);
+	fctx->fenceptr = fenceptr;
 	init_waitqueue_head(&fctx->event);
 	spin_lock_init(&fctx->spinlock);
 
@@ -35,7 +37,12 @@ void msm_fence_context_free(struct msm_fence_context *fctx)
 
 static inline bool fence_completed(struct msm_fence_context *fctx, uint32_t fence)
 {
-	return (int32_t)(fctx->completed_fence - fence) >= 0;
+	/*
+	 * Note: Check completed_fence first, as fenceptr is in a write-combine
+	 * mapping, so it will be more expensive to read.
+	 */
+	return (int32_t)(fctx->completed_fence - fence) >= 0 ||
+		(int32_t)(*fctx->fenceptr - fence) >= 0;
 }
 
 /* legacy path for WAIT_FENCE ioctl: */
diff --git a/drivers/gpu/drm/msm/msm_fence.h b/drivers/gpu/drm/msm/msm_fence.h
index 2d9af66dcca5..6ab97062ff1a 100644
--- a/drivers/gpu/drm/msm/msm_fence.h
+++ b/drivers/gpu/drm/msm/msm_fence.h
@@ -9,19 +9,52 @@
 
 #include "msm_drv.h"
 
+/**
+ * struct msm_fence_context - fence context for gpu
+ *
+ * Each ringbuffer has a single fence context, with the GPU writing an
+ * incrementing fence seqno at the end of each submit
+ */
 struct msm_fence_context {
 	struct drm_device *dev;
+	/** name: human readable name for fence timeline */
 	char name[32];
+	/** context: see dma_fence_context_alloc() */
 	unsigned context;
-	/* last_fence == completed_fence --> no pending work */
-	uint32_t last_fence;          /* last assigned fence */
-	uint32_t completed_fence;     /* last completed fence */
+
+	/**
+	 * last_fence:
+	 *
+	 * Last assigned fence, incremented each time a fence is created
+	 * on this fence context.  If last_fence == completed_fence,
+	 * there is no remaining pending work
+	 */
+	uint32_t last_fence;
+
+	/**
+	 * completed_fence:
+	 *
+	 * The last completed fence, updated from the CPU after interrupt
+	 * from GPU
+	 */
+	uint32_t completed_fence;
+
+	/**
+	 * fenceptr:
+	 *
+	 * The address that the GPU directly writes with completed fence
+	 * seqno.  This can be ahead of completed_fence.  We can peek at
+	 * this to see if a fence has already signaled but the CPU hasn't
+	 * gotten around to handling the irq and updating completed_fence
+	 */
+	volatile uint32_t *fenceptr;
+
 	wait_queue_head_t event;
 	spinlock_t spinlock;
 };
 
 struct msm_fence_context * msm_fence_context_alloc(struct drm_device *dev,
-		const char *name);
+		volatile uint32_t *fenceptr, const char *name);
 void msm_fence_context_free(struct msm_fence_context *fctx);
 
 int msm_wait_fence(struct msm_fence_context *fctx, uint32_t fence,
diff --git a/drivers/gpu/drm/msm/msm_ringbuffer.c b/drivers/gpu/drm/msm/msm_ringbuffer.c
index 4d2a2a4abef8..7e92d9532454 100644
--- a/drivers/gpu/drm/msm/msm_ringbuffer.c
+++ b/drivers/gpu/drm/msm/msm_ringbuffer.c
@@ -51,7 +51,7 @@ struct msm_ringbuffer *msm_ringbuffer_new(struct msm_gpu *gpu, int id,
 
 	snprintf(name, sizeof(name), "gpu-ring-%d", ring->id);
 
-	ring->fctx = msm_fence_context_alloc(gpu->dev, name);
+	ring->fctx = msm_fence_context_alloc(gpu->dev, &ring->memptrs->fence, name);
 
 	return ring;
 
-- 
2.31.1