drm/msm: Shadow current pointer in the ring until command is complete

Add a shadow pointer to track the current command being written into
the ring. Don't commit it as 'cur' until the command is submitted.
Because 'cur' is used to construct the software copy of the wptr this
ensures that somebody peeking in on the ring doesn't assume that a
command is inflight while it is being written. This isn't a huge deal
with a single ring (though technically the hangcheck could assume
the system is prematurely busy when it isn't) but it will be rather
important for preemption where the decision to preempt is based
on a non-empty ringbuffer. Without a shadow an aggressive preemption
scheme could assume that the ringbuffer is non empty and switch to it
before the CPU is done writing the command and boom.

Even though preemption won't be supported for all targets because of
the way the code is organized it is simpler to make this generic for
all targets. The extra load for non-preemption targets should be
minimal.

Change-Id: Ic0dedbad83247c3e77de6f4f24bbb97db10e5edd
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>

3 files changed, 16 insertions, 6 deletions

diff --git a/drivers/gpu/drm/msm/adreno/adreno_gpu.c b/drivers/gpu/drm/msm/adreno/adreno_gpu.c
index 8506ca2e9b8a..065b0f401a17 100644
--- a/drivers/gpu/drm/msm/adreno/adreno_gpu.c
+++ b/drivers/gpu/drm/msm/adreno/adreno_gpu.c
@@ -153,6 +153,7 @@ void adreno_recover(struct msm_gpu *gpu)
 			continue;
 
 		ring->cur = ring->start;
+		ring->next = ring->start;
 
 		/* reset completed fence seqno, discard anything pending: */
 		adreno_gpu->memptrs->fence[ring->id] =
@@ -259,12 +260,15 @@ void adreno_flush(struct msm_gpu *gpu, struct msm_ringbuffer *ring)
 	struct adreno_gpu *adreno_gpu = to_adreno_gpu(gpu);
 	uint32_t wptr;
 
+	/* Copy the shadow to the actual register */
+	ring->cur = ring->next;
+
 	/*
 	 * Mask the wptr value that we calculate to fit in the HW range. This is
 	 * to account for the possibility that the last command fit exactly into
 	 * the ringbuffer and rb->next hasn't wrapped to zero yet
 	 */
-	wptr = get_wptr(ring) % (MSM_GPU_RINGBUFFER_SZ >> 2);
+	wptr = (ring->cur - ring->start) % (MSM_GPU_RINGBUFFER_SZ >> 2);
 
 	/* ensure writes to ringbuffer have hit system memory: */
 	mb();
@@ -390,7 +394,8 @@ static uint32_t ring_freewords(struct msm_ringbuffer *ring)
 {
 	struct adreno_gpu *adreno_gpu = to_adreno_gpu(ring->gpu);
 	uint32_t size = MSM_GPU_RINGBUFFER_SZ >> 2;
-	uint32_t wptr = get_wptr(ring);
+	/* Use ring->next to calculate free size */
+	uint32_t wptr = ring->next - ring->start;
 	uint32_t rptr = get_rptr(adreno_gpu, ring);
 	return (rptr + (size - 1) - wptr) % size;
 }
diff --git a/drivers/gpu/drm/msm/msm_ringbuffer.c b/drivers/gpu/drm/msm/msm_ringbuffer.c
index 19ac38b2eba2..2dd6bc6c725c 100644
--- a/drivers/gpu/drm/msm/msm_ringbuffer.c
+++ b/drivers/gpu/drm/msm/msm_ringbuffer.c
@@ -43,6 +43,7 @@ struct msm_ringbuffer *msm_ringbuffer_new(struct msm_gpu *gpu, int id)
 
 	ring->start = msm_gem_vaddr_locked(ring->bo);
 	ring->end   = ring->start + (MSM_GPU_RINGBUFFER_SZ >> 2);
+	ring->next  = ring->start;
 	ring->cur   = ring->start;
 
 	return ring;
diff --git a/drivers/gpu/drm/msm/msm_ringbuffer.h b/drivers/gpu/drm/msm/msm_ringbuffer.h
index e40d130853e0..4e0b302501fa 100644
--- a/drivers/gpu/drm/msm/msm_ringbuffer.h
+++ b/drivers/gpu/drm/msm/msm_ringbuffer.h
@@ -24,7 +24,7 @@ struct msm_ringbuffer {
 	struct msm_gpu *gpu;
 	int id;
 	struct drm_gem_object *bo;
-	uint32_t *start, *end, *cur;
+	uint32_t *start, *end, *cur, *next;
 	uint64_t iova;
 	uint32_t submitted_fence;
 };
@@ -37,9 +37,13 @@ void msm_ringbuffer_destroy(struct msm_ringbuffer *ring);
 static inline void
 OUT_RING(struct msm_ringbuffer *ring, uint32_t data)
 {
-	if (ring->cur == ring->end)
-		ring->cur = ring->start;
-	*(ring->cur++) = data;
+	/*
+	 * ring->next points to the current command being written - it won't be
+	 * committed as ring->cur until the flush
+	 */
+	if (ring->next == ring->end)
+		ring->next = ring->start;
+	*(ring->next++) = data;
 }
 
 #endif /* __MSM_RINGBUFFER_H__ */