The Cathedral and the Bazaar is alive and well. I use my Apple Studio Display with a X1 Nano with Slackware 15 on it. But the brightness is still reserved for an old MacBook Air (ie you need to plugin the MB to the Studio Display, change the brightness, then switch back to the Linux machine).
But now Julius Zint has come up with a patch for the Linux Kernel and hopefully soon I won’t have to switch laptops to change the brightness of my display: Apple Studio Display Linux Driver Published To Control The Backlight - Phoronix
(source for the patch below)
diff --git a/drivers/video/backlight/Kconfig b/drivers/video/backlight/Kconfig
index 51387b1ef012..9383d402ebed 100644
--- a/drivers/video/backlight/Kconfig
+++ b/drivers/video/backlight/Kconfig
@@ -290,6 +290,14 @@ config BACKLIGHT_APPLE
If you have an Intel-based Apple say Y to enable a driver for its
backlight.
+config BACKLIGHT_APPLE_USB
+ tristate "Apple USB Backlight Driver"
+ depends on USB
+ help
+ If you have an external display from Apple that is attached via USB
+ say Y to enable a driver for its backlight. Currently it supports the
+ Apple Studio Display.
+
config BACKLIGHT_QCOM_WLED
tristate "Qualcomm PMIC WLED Driver"
select REGMAP
diff --git a/drivers/video/backlight/Makefile b/drivers/video/backlight/Makefile
index f72e1c3c59e9..c42880655113 100644
--- a/drivers/video/backlight/Makefile
+++ b/drivers/video/backlight/Makefile
@@ -23,6 +23,7 @@ obj-$(CONFIG_BACKLIGHT_ADP5520) += adp5520_bl.o
obj-$(CONFIG_BACKLIGHT_ADP8860) += adp8860_bl.o
obj-$(CONFIG_BACKLIGHT_ADP8870) += adp8870_bl.o
obj-$(CONFIG_BACKLIGHT_APPLE) += apple_bl.o
+obj-$(CONFIG_BACKLIGHT_APPLE_USB) += apple_bl_usb.o
obj-$(CONFIG_BACKLIGHT_AS3711) += as3711_bl.o
obj-$(CONFIG_BACKLIGHT_BD6107) += bd6107.o
obj-$(CONFIG_BACKLIGHT_CARILLO_RANCH) += cr_bllcd.o
diff --git a/drivers/video/backlight/apple_bl_usb.c b/drivers/video/backlight/apple_bl_usb.c
new file mode 100644
index 000000000000..b746b7822974
--- /dev/null
+++ b/drivers/video/backlight/apple_bl_usb.c
@@ -0,0 +1,264 @@
+// SPDX-License-Identifier: GPL-2.0 OR MIT
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/usb.h>
+#include <linux/backlight.h>
+#include <asm/byteorder.h>
+
+#define APPLE_STUDIO_DISPLAY_VENDOR_ID 0x05ac
+#define APPLE_STUDIO_DISPLAY_PRODUCT_ID 0x1114
+
+#define HID_GET_REPORT 0x01
+#define HID_SET_REPORT 0x09
+
+#define HID_REPORT_TYPE_FEATURE 0x0300
+
+struct apple_bl_usb_data {
+ struct usb_interface *usb_interface;
+ struct usb_device *usb_dev;
+};
+
+struct brightness_ctrl_message_data {
+ u8 unknown_1;
+ __le16 brightness;
+ u8 unkown_2[4];
+} __packed;
+
+void init_ctrl_msg_data(struct brightness_ctrl_message_data *msg)
+{
+ memset(msg, 0, sizeof(struct brightness_ctrl_message_data));
+ msg->unknown_1 = 0x01;
+}
+
+void set_ctrl_message_brightness(struct brightness_ctrl_message_data *msg,
+ u16 brightness_value)
+{
+ msg->brightness = cpu_to_le16(brightness_value + 400);
+}
+
+u16 get_ctrl_message_brightness(struct brightness_ctrl_message_data *msg)
+{
+ return le16_to_cpu(msg->brightness) - 400;
+}
+
+int apple_bl_usb_usb_get_brightness(struct usb_interface *interface,
+ struct usb_device *usb_dev,
+ int *brightness)
+{
+ int err;
+ u16 interface_nr;
+ int msg_data_size;
+ struct brightness_ctrl_message_data *msg_data;
+
+ msg_data_size = sizeof(struct brightness_ctrl_message_data);
+ msg_data = kzalloc(msg_data_size, GFP_KERNEL);
+ memset(msg_data, 0x00, msg_data_size);
+ interface_nr = interface->cur_altsetting->desc.bInterfaceNumber;
+
+ err = usb_control_msg(usb_dev,
+ usb_rcvctrlpipe(usb_dev, 0),
+ HID_GET_REPORT,
+ USB_DIR_IN | USB_TYPE_CLASS | USB_RECIP_INTERFACE,
+ /* wValue: HID-Report Type and Report ID */
+ HID_REPORT_TYPE_FEATURE | 0x01,
+ interface_nr /* wIndex */,
+ msg_data,
+ msg_data_size,
+ HZ);
+ if (err < 0) {
+ dev_err(&interface->dev,
+ "get: usb control message err: %d\n",
+ err);
+ }
+ *brightness = get_ctrl_message_brightness(msg_data);
+ kfree(msg_data);
+ dev_dbg(&interface->dev, "get brightness: %d\n", *brightness);
+ return 0;
+}
+
+int apple_bl_usb_usb_set_brightness(struct usb_interface *interface,
+ struct usb_device *usb_dev,
+ int brightness)
+{
+ int err;
+ u16 interface_nr;
+ struct brightness_ctrl_message_data *msg_data;
+
+ msg_data = kzalloc(sizeof(struct brightness_ctrl_message_data), GFP_KERNEL);
+ interface_nr = interface->cur_altsetting->desc.bInterfaceNumber;
+ init_ctrl_msg_data(msg_data);
+ set_ctrl_message_brightness(msg_data, brightness);
+
+ err = usb_control_msg(usb_dev,
+ usb_sndctrlpipe(usb_dev, 0),
+ HID_SET_REPORT,
+ USB_DIR_OUT | USB_TYPE_CLASS | USB_RECIP_INTERFACE,
+ /* wValue: HID-Report Type and Report ID */
+ HID_REPORT_TYPE_FEATURE | 0x01,
+ interface_nr /* wIndex */,
+ msg_data,
+ sizeof(struct brightness_ctrl_message_data),
+ HZ);
+ kfree(msg_data);
+ if (err < 0) {
+ dev_err(&interface->dev,
+ "set: usb control message err: %d\n",
+ err);
+ return err;
+ }
+ dev_dbg(&interface->dev, "set brightness: %d\n", brightness);
+ return 0;
+}
+
+int apple_bl_usb_check_fb(struct backlight_device *bd, struct fb_info *info)
+{
+ dev_info(&bd->dev, "check fb\n");
+ return 0;
+}
+
+int apple_bl_usb_get_brightness(struct backlight_device *bl)
+{
+ int ret;
+ struct apple_bl_usb_data *data;
+ int hw_brightness;
+
+ data = bl_get_data(bl);
+ ret = apple_bl_usb_usb_get_brightness(data->usb_interface,
+ data->usb_dev,
+ &hw_brightness);
+ if (!ret)
+ ret = hw_brightness;
+
+ return ret;
+}
+
+int apple_bl_usb_update_status(struct backlight_device *bl)
+{
+ int err;
+ struct apple_bl_usb_data *data;
+
+ data = bl_get_data(bl);
+ err = apple_bl_usb_usb_set_brightness(data->usb_interface,
+ data->usb_dev,
+ bl->props.brightness);
+ return err;
+}
+
+static const struct backlight_ops apple_bl_usb_ops = {
+ .update_status = apple_bl_usb_update_status,
+ .get_brightness = apple_bl_usb_get_brightness,
+ .check_fb = apple_bl_usb_check_fb,
+};
+
+static void apple_bl_usb_disconnect(struct usb_interface *interface)
+{
+ struct backlight_device *bl;
+
+ dev_dbg(&interface->dev, "disconnect\n");
+
+ bl = usb_get_intfdata(interface);
+ usb_set_intfdata(interface, NULL);
+ backlight_device_unregister(bl);
+}
+
+static int apple_bl_usb_probe(struct usb_interface *interface,
+ const struct usb_device_id *id)
+{
+ struct backlight_properties props;
+ struct backlight_device *bl;
+ struct usb_device *usb_dev;
+ struct device *dev;
+ struct apple_bl_usb_data *data;
+ int brightness_interface_nr;
+
+ dev_dbg(&interface->dev, "probe\n");
+
+ dev = &interface->dev;
+ usb_dev = interface_to_usbdev(interface);
+
+ switch (usb_dev->config->desc.bConfigurationValue) {
+ case 1:
+ brightness_interface_nr = 0x7;
+ break;
+ case 2:
+ brightness_interface_nr = 0x9;
+ break;
+ case 3:
+ brightness_interface_nr = 0xc;
+ break;
+ default:
+ dev_err(dev,
+ "unexpected configuration value: %d\n",
+ usb_dev->config->desc.bConfigurationValue);
+ return -EINVAL;
+ }
+
+ if (interface->cur_altsetting->desc.bInterfaceNumber != brightness_interface_nr)
+ return -ENODEV;
+
+ data = devm_kzalloc(dev,
+ sizeof(struct apple_bl_usb_data),
+ GFP_KERNEL);
+ if (IS_ERR(data)) {
+ dev_err(dev, "failed to allocate memory\n");
+ return PTR_ERR(bl);
+ }
+ data->usb_interface = interface;
+ data->usb_dev = usb_dev;
+
+ // Valid brightness values for the apple studio display range from 400
+ // to 60000. Since the backlight subsystem´s brightness value starts
+ // from 0, we use 0 to 59600 and offset it by the minimum value.
+ memset(&props, 0, sizeof(props));
+ props.type = BACKLIGHT_RAW;
+ props.max_brightness = 59600;
+
+ bl = backlight_device_register("apple_studio_display",
+ dev,
+ data,
+ &apple_bl_usb_ops,
+ &props);
+ if (IS_ERR(bl)) {
+ dev_err(dev, "failed to register backlight\n");
+ return PTR_ERR(bl);
+ }
+ usb_set_intfdata(interface, bl);
+ return 0;
+}
+
+static int apple_bl_usb_suspend(struct usb_interface *interface,
+ pm_message_t message)
+{
+ dev_dbg(&interface->dev, "suspend\n");
+ return 0;
+}
+
+static int apple_bl_usb_resume(struct usb_interface *interface)
+{
+ dev_dbg(&interface->dev, "resume\n");
+ return 0;
+}
+
+static const struct usb_device_id id_table[] = {
+ {
+ .idVendor = APPLE_STUDIO_DISPLAY_VENDOR_ID,
+ .idProduct = APPLE_STUDIO_DISPLAY_PRODUCT_ID,
+ },
+ {},
+};
+MODULE_DEVICE_TABLE(usb, id_table);
+
+static struct usb_driver usb_asdbl_driver = {
+ .name = "apple_bl_usb",
+ .probe = apple_bl_usb_probe,
+ .disconnect = apple_bl_usb_disconnect,
+ .id_table = id_table,
+ .suspend = apple_bl_usb_suspend,
+ .resume = apple_bl_usb_resume,
+ .reset_resume = apple_bl_usb_resume
+};
+module_usb_driver(usb_asdbl_driver);
+
+MODULE_AUTHOR("Julius Zint <julius@zint.sh>");
+MODULE_LICENSE("Dual MIT/GPL");
+MODULE_DESCRIPTION("Backlight control for USB-C Thunderbolt Apple displays");