drm/amd/display: Skip dmub memory flush when not needed

[WHY&HOW]
Readback is only necessary when loaded via CPU.
Reviewed-by: Chris Park <chris.park@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dmub/dmub_srv.h

@@ -142,6 +142,13 @@ enum dpia_notify_bw_alloc_status {
 	DPIA_BW_ALLOC_CAPS_CHANGED
 };
 
+/* enum dmub_memory_access_type - memory access method */
+enum dmub_memory_access_type {
+	DMUB_MEMORY_ACCESS_DEFAULT,
+	DMUB_MEMORY_ACCESS_CPU = DMUB_MEMORY_ACCESS_DEFAULT,
+	DMUB_MEMORY_ACCESS_DMA
+};
+
 /**
  * struct dmub_region - dmub hw memory region
  * @base: base address for region, must be 256 byte aligned
@@ -264,6 +271,7 @@ struct dmub_srv_hw_params {
 	bool dpia_hpd_int_enable_supported;
 	bool disable_clock_gate;
 	bool disallow_dispclk_dppclk_ds;
+	enum dmub_memory_access_type mem_access_type;
 };
 
 /**

drivers/gpu/drm/amd/display/dmub/src/dmub_srv.c

@@ -564,6 +564,7 @@ enum dmub_status dmub_srv_hw_init(struct dmub_srv *dmub,
 		 * DMCUB when backdoor loading if the write from x86 hasn't been
 		 * flushed yet. This only occurs in backdoor loading.
 		 */
+		if (params->mem_access_type == DMUB_MEMORY_ACCESS_CPU)
 			dmub_flush_buffer_mem(inst_fb);
 
 		if (params->fw_in_system_memory && dmub->hw_funcs.backdoor_load_zfb_mode)