drm/amdgpu: optimize amdgpu_device_vram_access a bit.

Only write the _HI register when necessary.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Jonathan Kim <Jonathan.Kim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -183,20 +183,25 @@ bool amdgpu_device_supports_baco(struct drm_device *dev)
 void amdgpu_device_vram_access(struct amdgpu_device *adev, loff_t pos,
 			       uint32_t *buf, size_t size, bool write)
 {
-	uint64_t last;
 	unsigned long flags;
+	uint32_t hi = ~0;
+	uint64_t last;
+
+	spin_lock_irqsave(&adev->mmio_idx_lock, flags);
+	for (last = pos + size; pos < last; pos += 4) {
+		uint32_t tmp = pos >> 31;
 
-	last = size - 4;
-	for (last += pos; pos <= last; pos += 4) {
-		spin_lock_irqsave(&adev->mmio_idx_lock, flags);
 		WREG32_NO_KIQ(mmMM_INDEX, ((uint32_t)pos) | 0x80000000);
-		WREG32_NO_KIQ(mmMM_INDEX_HI, pos >> 31);
+		if (tmp != hi) {
+			WREG32_NO_KIQ(mmMM_INDEX_HI, tmp);
+			hi = tmp;
+		}
 		if (write)
 			WREG32_NO_KIQ(mmMM_DATA, *buf++);
 		else
 			*buf++ = RREG32_NO_KIQ(mmMM_DATA);
-		spin_unlock_irqrestore(&adev->mmio_idx_lock, flags);
 	}
+	spin_unlock_irqrestore(&adev->mmio_idx_lock, flags);
 }
 
 /*