[PATCH v3] drm/amdgpu: Fix missing comment for mb() in 'amdgpu_device_aper_access'

[Srinivasan Shanmugam]

This patch adds the missing code comment for memory barrier

WARNING: memory barrier without comment
+                       mb();

WARNING: memory barrier without comment
+                       mb();

Cc: Guchun Chen <guchun.chen at amd.com>
Cc: Christian König <christian.koenig at amd.com>
Cc: Alex Deucher <alexander.deucher at amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan at amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam at amd.com>
---
v3:

Corrected title message 

 drivers/gpu/drm/amd/amdgpu/amdgpu_device.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
index 4f818f13dc9d..bb65e12ed027 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
@@ -305,10 +305,17 @@ size_t amdgpu_device_aper_access(struct amdgpu_device *adev, loff_t pos,
 
 		if (write) {
 			memcpy_toio(addr, buf, count);
+			/* Make sure contents of system memory are copied/flushed
+			 * without messing up order of writes to PCI device I/O memory
+			 */
 			mb();
 			amdgpu_device_flush_hdp(adev, NULL);
 		} else {
 			amdgpu_device_invalidate_hdp(adev, NULL);
+			/* Make sure contents of system memory drained/invalidated
+			 * before copying contents from PCI device I/O memory
+			 * to system memory
+			 */
 			mb();
 			memcpy_fromio(buf, addr, count);
 		}
-- 
2.25.1