Currently the value written to CP_RB_WPTR is calculated on the fly as
(rb->next - rb->start). But as the code is designed rb->next is wrapped
before writing the commands so if a series of commands happened to
fit perfectly in the ringbuffer, rb->next would end up being equal to
rb->size / 4 and thus result in an out of bounds address to CP_RB_WPTR.

The easiest way to fix this is to mask WPTR when writing it to the
hardware; it makes the hardware happy and the rest of the ringbuffer
math appears to work and there isn't any point in upsetting anything.

Signed-off-by: Jordan Crouse <jcro...@codeaurora.org>
---
 drivers/gpu/drm/msm/adreno/adreno_gpu.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/msm/adreno/adreno_gpu.c 
b/drivers/gpu/drm/msm/adreno/adreno_gpu.c
index f386f46..b45481a 100644
--- a/drivers/gpu/drm/msm/adreno/adreno_gpu.c
+++ b/drivers/gpu/drm/msm/adreno/adreno_gpu.c
@@ -210,7 +210,14 @@ void adreno_submit(struct msm_gpu *gpu, struct 
msm_gem_submit *submit,
 void adreno_flush(struct msm_gpu *gpu)
 {
        struct adreno_gpu *adreno_gpu = to_adreno_gpu(gpu);
-       uint32_t wptr = get_wptr(gpu->rb);
+       uint32_t wptr;
+
+       /*
+        * 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(gpu->rb) & ((rb->size / 4) - 1);
 
        /* ensure writes to ringbuffer have hit system memory: */
        mb();
-- 
1.9.1

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