amdgpu/gmc_v9: Warn if SDPIF_MMIO_CNTRL_0 is not set

With IOMMU enabled, if SDPIF_MMIO_CNTRL_0 is not set
appropriately the system hangs without any trace
during S3.

To ease debug and to ensure that the failure, if any,
was caused by a race conditions that disabled write access to
SDPIF_MMIO_CNTRL_0 register, warn the user about it.
Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c

@@ -1545,8 +1545,11 @@ static void gmc_v9_0_init_golden_registers(struct amdgpu_device *adev)
  */
 void gmc_v9_0_restore_registers(struct amdgpu_device *adev)
 {
-	if (adev->asic_type == CHIP_RAVEN)
+	if (adev->asic_type == CHIP_RAVEN) {
 		WREG32_SOC15(DCE, 0, mmDCHUBBUB_SDPIF_MMIO_CNTRL_0, adev->gmc.sdpif_register);
+		WARN_ON(adev->gmc.sdpif_register !=
+			RREG32_SOC15(DCE, 0, mmDCHUBBUB_SDPIF_MMIO_CNTRL_0));
+	}
 }
 
 /**