Re: [PATCH] drm/msm: Fix fence rollover issue

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On 15/06/2022 19:24, Rob Clark wrote:
From: Rob Clark <robdclark@xxxxxxxxxxxx>

And while we are at it, let's start the fence counter close to the
rollover point so that if issues slip in, they are more obvious.

Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>

Should it also have

Fixes: fde5de6cb461 ("drm/msm: move fence code to it's own file")

Or maybe

Fixes: 5f3aee4ceb5b ("drm/msm: Handle fence rollover")

Otherwise:

Reviewed: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>


---
  drivers/gpu/drm/msm/msm_fence.c | 13 +++++++++++--
  1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/msm/msm_fence.c b/drivers/gpu/drm/msm/msm_fence.c
index 3df255402a33..a35a6746c7cd 100644
--- a/drivers/gpu/drm/msm/msm_fence.c
+++ b/drivers/gpu/drm/msm/msm_fence.c
@@ -28,6 +28,14 @@ msm_fence_context_alloc(struct drm_device *dev, volatile uint32_t *fenceptr,
  	fctx->fenceptr = fenceptr;
  	spin_lock_init(&fctx->spinlock);
+ /*
+	 * Start out close to the 32b fence rollover point, so we can
+	 * catch bugs with fence comparisons.
+	 */
+	fctx->last_fence = 0xffffff00;
+	fctx->completed_fence = fctx->last_fence;
+	*fctx->fenceptr = fctx->last_fence;

This looks like a debugging hack. But probably it's fine to have it, as it wouldn't cause any side effects.

+
  	return fctx;
  }
@@ -46,11 +54,12 @@ bool msm_fence_completed(struct msm_fence_context *fctx, uint32_t fence)
  		(int32_t)(*fctx->fenceptr - fence) >= 0;
  }
-/* called from workqueue */
+/* called from irq handler */
  void msm_update_fence(struct msm_fence_context *fctx, uint32_t fence)
  {
  	spin_lock(&fctx->spinlock);
-	fctx->completed_fence = max(fence, fctx->completed_fence);
+	if (fence_after(fence, fctx->completed_fence))
+		fctx->completed_fence = fence;
  	spin_unlock(&fctx->spinlock);
  }


--
With best wishes
Dmitry



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