Re: [PATCH] drm/amdgpu: Use READ_ONCE() when reading the values in 'sdma_v4_4_2_ring_get_rptr'

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Am 04.08.23 um 07:46 schrieb Srinivasan Shanmugam:
Instead of declaring pointers use READ_ONCE(), when accessing those
values to make sure that the compiler doesn't voilate any cache
coherences

That commit message is a bit confusing and not 100% technically correct.

The compiler is not causing any cache coherency issues, but potentially re-ordering things or reading the value multiple times.

Just write something like "Use READ_ONCE() instead of declaring the pointer volatile.". The background explanation would exceed the information suitable for a commit message anyway.

Apart from that looks good to me,
Christian.


Cc: Guchun Chen <guchun.chen@xxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
Cc: Alex Deucher <alexander.deucher@xxxxxxx>
Cc: "Pan, Xinhui" <Xinhui.Pan@xxxxxxx>
Cc: Le Ma <le.ma@xxxxxxx>
Cc: Hawking Zhang <Hawking.Zhang@xxxxxxx>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@xxxxxxx>
---
  drivers/gpu/drm/amd/amdgpu/sdma_v4_4_2.c | 8 ++++----
  1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/sdma_v4_4_2.c b/drivers/gpu/drm/amd/amdgpu/sdma_v4_4_2.c
index f413898dda37..267c1b7b8dcd 100644
--- a/drivers/gpu/drm/amd/amdgpu/sdma_v4_4_2.c
+++ b/drivers/gpu/drm/amd/amdgpu/sdma_v4_4_2.c
@@ -154,13 +154,13 @@ static int sdma_v4_4_2_init_microcode(struct amdgpu_device *adev)
   */
  static uint64_t sdma_v4_4_2_ring_get_rptr(struct amdgpu_ring *ring)
  {
-	u64 *rptr;
+	u64 rptr;
/* XXX check if swapping is necessary on BE */
-	rptr = ((u64 *)&ring->adev->wb.wb[ring->rptr_offs]);
+	rptr = READ_ONCE(*((u64 *)&ring->adev->wb.wb[ring->rptr_offs]));
- DRM_DEBUG("rptr before shift == 0x%016llx\n", *rptr);
-	return ((*rptr) >> 2);
+	DRM_DEBUG("rptr before shift == 0x%016llx\n", rptr);
+	return rptr >> 2;
  }
/**




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