[PATCH v3 3/3] drm/amdgpu: read back register after written for VCN v5.0.0

[Dong, Ruijing]

[AMD Official Use Only - AMD Internal Distribution Only]

The series is
Reviewed-by: Ruijing Dong <ruijing.dong at amd.com>

-----Original Message-----
From: Wu, David <David.Wu3 at amd.com>
Sent: Tuesday, May 13, 2025 3:22 PM
To: amd-gfx at lists.freedesktop.org; Koenig, Christian <Christian.Koenig at amd.com>
Cc: Deucher, Alexander <Alexander.Deucher at amd.com>; Liu, Leo <Leo.Liu at amd.com>; Jiang, Sonny <Sonny.Jiang at amd.com>; Dong, Ruijing <Ruijing.Dong at amd.com>; Limonciello, Mario <Mario.Limonciello at amd.com>
Subject: [PATCH v3 3/3] drm/amdgpu: read back register after written for VCN v5.0.0

V3: patch for VCN v5.0.0 only

Similar to the previous changes made for VCN v4.0.5, the addition of register read-back support in VCN v5.0.0 is intended to prevent potential race conditions, even though such issues have not been observed yet.
This change ensures consistency across different VCN variants and helps avoid similar issues on newer or closely related GPUs. The overhead introduced by this read-back is negligible.

Signed-off-by: David (Ming Qiang) Wu <David.Wu3 at amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello at amd.com>
---
 drivers/gpu/drm/amd/amdgpu/vcn_v5_0_0.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/vcn_v5_0_0.c b/drivers/gpu/drm/amd/amdgpu/vcn_v5_0_0.c
index 27dcc6f37a73..77c27a317e4c 100644
--- a/drivers/gpu/drm/amd/amdgpu/vcn_v5_0_0.c
+++ b/drivers/gpu/drm/amd/amdgpu/vcn_v5_0_0.c
@@ -794,6 +794,10 @@ static int vcn_v5_0_0_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