Would it make sense to include a link to a better explanation of the
underlying issue? E.g. https://lwn.net/Articles/624126/?
Regards,
Felix
On 2023-08-21 07:23, Christian König wrote:
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 <[email protected]>
Cc: Christian König <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: "Pan, Xinhui" <[email protected]>
Cc: Le Ma <[email protected]>
Cc: Hawking Zhang <[email protected]>
Signed-off-by: Srinivasan Shanmugam <[email protected]>
---
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;
}
/**
