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

["Christian König" <christian.koenig@amd.com>]

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@amd.com>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Alex Deucher <alexander.deucher@amd.com>
> Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
> Cc: Le Ma <le.ma@amd.com>
> Cc: Hawking Zhang <Hawking.Zhang@amd.com>
> Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
> ---
>   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;
>   }
>   
>   /**