drm/amd/display: Implement backlight_ops.get_brightness

[Why]
This hook that is supposed to read the actual backlight value
is used in a few places throughout the kernel to setup or force
update on backlight

[How]
Create a dc function that calls the existing abm function, and
call that function from amdgpu

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -1320,7 +1320,12 @@ static int amdgpu_dm_backlight_update_status(struct backlight_device *bd)
 
 static int amdgpu_dm_backlight_get_brightness(struct backlight_device *bd)
 {
-	return bd->props.brightness;
+	struct amdgpu_display_manager *dm = bl_get_data(bd);
+	int ret = dc_link_get_backlight_level(dm->backlight_link);
+
+	if (ret == DC_ERROR_UNEXPECTED)
+		return bd->props.brightness;
+	return ret;
 }
 
 static const struct backlight_ops amdgpu_dm_backlight_ops = {

drivers/gpu/drm/amd/display/dc/core/dc_link.c

@@ -2026,6 +2026,15 @@ enum dc_status dc_link_validate_mode_timing(
 	return DC_OK;
 }
 
+int dc_link_get_backlight_level(const struct dc_link *link)
+{
+	struct abm *abm = link->ctx->dc->res_pool->abm;
+
+	if (abm == NULL || abm->funcs->get_current_backlight_8_bit == NULL)
+		return DC_ERROR_UNEXPECTED;
+
+	return (int) abm->funcs->get_current_backlight_8_bit(abm);
+}
 
 bool dc_link_set_backlight_level(const struct dc_link *link, uint32_t level,
 		uint32_t frame_ramp, const struct dc_stream_state *stream)

drivers/gpu/drm/amd/display/dc/dc_link.h

@@ -141,6 +141,8 @@ static inline struct dc_link *dc_get_link_at_index(struct dc *dc, uint32_t link_
 bool dc_link_set_backlight_level(const struct dc_link *dc_link, uint32_t level,
 		uint32_t frame_ramp, const struct dc_stream_state *stream);
 
+int dc_link_get_backlight_level(const struct dc_link *dc_link);
+
 bool dc_link_set_abm_disable(const struct dc_link *dc_link);
 
 bool dc_link_set_psr_enable(const struct dc_link *dc_link, bool enable, bool wait);