[PATCH 15/17] 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 39ff842..a2777d8 100644
--- a/drivers/gpu/drm/msm/adreno/adreno_gpu.c
+++ b/drivers/gpu/drm/msm/adreno/adreno_gpu.c
@@ -82,6 +82,7 @@ int adreno_hw_init(struct msm_gpu *gpu)
 		}
 
 		ring->cur = ring->start;
+		ring->next = ring->start;
 
 		/* reset completed fence seqno: */
 		ring->memptrs->fence = ring->seqno;
@@ -226,12 +227,15 @@ 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();
@@ -343,7 +347,8 @@ 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 a74d66a..899145b 100644
--- a/drivers/gpu/drm/msm/msm_ringbuffer.c
+++ b/drivers/gpu/drm/msm/msm_ringbuffer.c
@@ -45,6 +45,7 @@ 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;
 
 	ring->memptrs = memptrs;
diff --git a/drivers/gpu/drm/msm/msm_ringbuffer.h b/drivers/gpu/drm/msm/msm_ringbuffer.h
index 5907063f..8cff38a 100644
--- a/drivers/gpu/drm/msm/msm_ringbuffer.h
+++ b/drivers/gpu/drm/msm/msm_ringbuffer.h
@@ -32,7 +32,7 @@ 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;
 	struct list_head submits;
 	uint64_t iova;
 	uint32_t seqno;
@@ -50,9 +50,13 @@ struct msm_ringbuffer *msm_ringbuffer_new(struct msm_gpu *gpu, int id,
 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