[PATCH 09/11] drm/msm: Shadow current pointer in the ring until command is complete

[Jordan Crouse]

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.

Signed-off-by: Jordan Crouse <jcrouse at codeaurora.org>
---
 drivers/gpu/drm/msm/adreno/adreno_gpu.c |  9 +++++++--
 drivers/gpu/drm/msm/msm_ringbuffer.c    |  1 +
 drivers/gpu/drm/msm/msm_ringbuffer.h    | 12 ++++++++----
 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 21c839f..b8c11a0 100644
--- a/drivers/gpu/drm/msm/adreno/adreno_gpu.c
+++ b/drivers/gpu/drm/msm/adreno/adreno_gpu.c
@@ -144,6 +144,7 @@ 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] = ring->completed_fence;
@@ -240,12 +241,15 @@ 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 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();
@@ -366,7 +370,8 @@ 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 2ab31c7..b885979 100644
--- a/drivers/gpu/drm/msm/msm_ringbuffer.c
+++ b/drivers/gpu/drm/msm/msm_ringbuffer.c
@@ -47,6 +47,7 @@ struct msm_ringbuffer *msm_ringbuffer_new(struct msm_gpu *gpu, int id)
 		goto fail;
 	}
 	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 4eb05fe..865b21a 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;
 	/* last_fence == completed_fence --> no pending work */
 	uint32_t last_fence;
@@ -39,9 +39,13 @@ struct msm_ringbuffer {
 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__ */
-- 
1.9.1