drm/i915/dsi: Read/write proper brightness value via MIPI DCS command

Driver has to swap the endian before send brightness level value
to tcon.

v2: Use __be16 instead of u16 to fix sparse warning.
v3: Send one or two bytes brightness value depend on the precision.
v4: get data length of brightness value more easily.
Reported-by: kernel test robot <lkp@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Cooper Chiou <cooper.chiou@intel.com>
Cc: William Tseng <william.tseng@intel.com>
Signed-off-by: Lee Shawn C <shawn.c.lee@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210908115607.9633-6-shawn.c.lee@intel.com

drivers/gpu/drm/i915/display/intel_dsi_dcs_backlight.c

@@ -47,33 +47,42 @@ static u32 dcs_get_backlight(struct intel_connector *connector, enum pipe unused
 {
 	struct intel_encoder *encoder = intel_attached_encoder(connector);
 	struct intel_dsi *intel_dsi = enc_to_intel_dsi(encoder);
+	struct intel_panel *panel = &connector->panel;
 	struct mipi_dsi_device *dsi_device;
-	u8 data = 0;
+	u8 data[2] = {};
 	enum port port;
+	size_t len = panel->backlight.max > U8_MAX ? 2 : 1;
 
-	/* FIXME: Need to take care of 16 bit brightness level */
 	for_each_dsi_port(port, intel_dsi->dcs_backlight_ports) {
 		dsi_device = intel_dsi->dsi_hosts[port]->device;
 		mipi_dsi_dcs_read(dsi_device, MIPI_DCS_GET_DISPLAY_BRIGHTNESS,
-				  &data, sizeof(data));
+				  &data, len);
 		break;
 	}
 
-	return data;
+	return (data[1] << 8) | data[0];
 }
 
 static void dcs_set_backlight(const struct drm_connector_state *conn_state, u32 level)
 {
 	struct intel_dsi *intel_dsi = enc_to_intel_dsi(to_intel_encoder(conn_state->best_encoder));
+	struct intel_panel *panel = &to_intel_connector(conn_state->connector)->panel;
 	struct mipi_dsi_device *dsi_device;
-	u8 data = level;
+	u8 data[2] = {};
 	enum port port;
+	size_t len = panel->backlight.max > U8_MAX ? 2 : 1;
+
+	if (len == 1) {
+		data[0] = level;
+	} else {
+		data[0] = level >> 8;
+		data[1] = level;
+	}
 
-	/* FIXME: Need to take care of 16 bit brightness level */
 	for_each_dsi_port(port, intel_dsi->dcs_backlight_ports) {
 		dsi_device = intel_dsi->dsi_hosts[port]->device;
 		mipi_dsi_dcs_write(dsi_device, MIPI_DCS_SET_DISPLAY_BRIGHTNESS,
-				   &data, sizeof(data));
+				   &data, len);
 	}
 }