[PATCH V2 1/2] drm/amdgpu: read back register after written for VCN v4.0.5

[Alex Deucher]

On Tue, May 13, 2025 at 2:23 PM David (Ming Qiang) Wu <David.Wu3 at amd.com> wrote:
>
> V2: not to add extra read-back in vcn_v4_0_5_start as there is a
>     read-back already. New comment for better understanding.
>
> On VCN v4.0.5 there is a race condition where the WPTR is not
> updated after starting from idle when doorbell is used. The read-back
> of regVCN_RB1_DB_CTRL register after written is to ensure the
> doorbell_index is updated before it can work properly.
>
> Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/12528
> Cc: stable at vger.kernel.org
>
> Signed-off-by: David (Ming Qiang) Wu <David.Wu3 at amd.com>
> Reviewed-by: Mario Limonciello <mario.limonciello at amd.com>
> Tested-by: Mario Limonciello <mario.limonciello at amd.com>

Reviewed-by: Alex Deucher <alexander.deucher at amd.com>

> ---
>  drivers/gpu/drm/amd/amdgpu/vcn_v4_0_5.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/vcn_v4_0_5.c b/drivers/gpu/drm/amd/amdgpu/vcn_v4_0_5.c
> index ed00d35039c13..e55b76d71367d 100644
> --- a/drivers/gpu/drm/amd/amdgpu/vcn_v4_0_5.c
> +++ b/drivers/gpu/drm/amd/amdgpu/vcn_v4_0_5.c
> @@ -1034,6 +1034,10 @@ static int vcn_v4_0_5_start_dpg_mode(struct amdgpu_vcn_inst *vinst,
>                         ring->doorbell_index << VCN_RB1_DB_CTRL__OFFSET__SHIFT |
>                         VCN_RB1_DB_CTRL__EN_MASK);
>
> +       /* Keeping one read-back to ensure all register writes are done, otherwise
> +        * it may introduce race conditions */
> +       RREG32_SOC15(VCN, inst_idx, regVCN_RB1_DB_CTRL);
> +
>         return 0;
>  }
>
> --
> 2.34.1
>