drm/amdgpu: Add module parameter for specifying default ABM level

[Why]
It's non trivial to configure or specify an ABM reduction level for
userspace outside of X. There is also no method to specify the default
ABM value at boot time.

A parameter should be added to configure this.

[How]
Expose a module parameter that can specify the default ABM level to
use for eDP connectors on DC enabled hardware that loads the DMCU
firmware.

The default is still disabled (0), but levels can range from 1-4. Levels
control how much the backlight can be reduced, with being the least
amount of reduction and four being the most reduction.
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: David Francis <david.francis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu.h

@@ -154,6 +154,7 @@ extern int amdgpu_gpu_recovery;
 extern int amdgpu_emu_mode;
 extern uint amdgpu_smu_memory_pool_size;
 extern uint amdgpu_dc_feature_mask;
+extern uint amdgpu_dm_abm_level;
 extern struct amdgpu_mgpu_info mgpu_info;
 extern int amdgpu_ras_enable;
 extern uint amdgpu_ras_mask;

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

@@ -678,6 +678,22 @@ MODULE_PARM_DESC(hws_gws_support, "MEC FW support gws barriers (false = not supp
 MODULE_PARM_DESC(dcfeaturemask, "all stable DC features enabled (default))");
 module_param_named(dcfeaturemask, amdgpu_dc_feature_mask, uint, 0444);
 
+/**
+ * DOC: abmlevel (uint)
+ * Override the default ABM (Adaptive Backlight Management) level used for DC
+ * enabled hardware. Requires DMCU to be supported and loaded.
+ * Valid levels are 0-4. A value of 0 indicates that ABM should be disabled by
+ * default. Values 1-4 control the maximum allowable brightness reduction via
+ * the ABM algorithm, with 1 being the least reduction and 4 being the most
+ * reduction.
+ *
+ * Defaults to 0, or disabled. Userspace can still override this level later
+ * after boot.
+ */
+uint amdgpu_dm_abm_level = 0;
+MODULE_PARM_DESC(abmlevel, "ABM level (0 = off (default), 1-4 = backlight reduction level) ");
+module_param_named(abmlevel, amdgpu_dm_abm_level, uint, 0444);
+
 static const struct pci_device_id pciidlist[] = {
 #ifdef  CONFIG_DRM_AMDGPU_SI
 	{0x1002, 0x6780, PCI_ANY_ID, PCI_ANY_ID, 0, 0, CHIP_TAHITI},