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

[David (Ming Qiang) Wu]

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>
---
 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