drm/msm: Ensure that the hardware write pointer is valid

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 <jcrouse@codeaurora.org>
[squash in is_power_of_2() check]
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/adreno/adreno_gpu.c

@@ -213,7 +213,14 @@ 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) & ((gpu->rb->size / 4) - 1);
 
 	/* ensure writes to ringbuffer have hit system memory: */
 	mb();

drivers/gpu/drm/msm/msm_ringbuffer.c

@@ -23,7 +23,8 @@ struct msm_ringbuffer *msm_ringbuffer_new(struct msm_gpu *gpu, int size)
 	struct msm_ringbuffer *ring;
 	int ret;
 
-	size = ALIGN(size, 4);   /* size should be dword aligned */
+	if (WARN_ON(!is_power_of_2(size)))
+		return ERR_PTR(-EINVAL);
 
 	ring = kzalloc(sizeof(*ring), GFP_KERNEL);
 	if (!ring) {