Re: [PATCH 1/1] backlight: apple_bl_usb: Add Apple Studio Display support

From: Sam Ravnborg
Date: Sat Jul 01 2023 - 18:02:02 EST

Hi Julius.

Thanks for posting this. I few nits in the following where you as the
author decide what to ignore and what to update.

Sam


On Sat, Jul 01, 2023 at 02:08:03PM +0200, Julius Zint wrote:
> The Apple Studio Display does not have any physical buttons and the only
> way to get or set the brightness is by sending USB control transfers to a
> HID device exposed by the display.
>
> These control transfers take the form of a HID_(GET|SET)_REPORT request
> and the payload looks like this:
>
> struct brightness_ctrl_message_data {
> u8 unknown_1;
> __le16 brightness;
> u8 unkown_2[4];
> } __packed;
>
> When compiled as a module this driver needs to be part of the early boot
> environment, otherwise the generic USB HID driver will claim the device.

I hope someone else can help here, as I have no clue.

>
> Signed-off-by: Julius Zint <julius@xxxxxxx>
> ---
> drivers/video/backlight/Kconfig | 8 +
> drivers/video/backlight/Makefile | 1 +
> drivers/video/backlight/apple_bl_usb.c | 264 +++++++++++++++++++++++++
> 3 files changed, 273 insertions(+)
> create mode 100644 drivers/video/backlight/apple_bl_usb.c
>
> 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;

A different way to set the brightness value could be:
struct brightness_ctrl_message_data {
u8 cmd;
u8[2] brightness;
u8 unknown[4];
} __packed;

static void set_ctrl_message_brightness(struct brightness_ctrl_message_data *msg,
u16 brightness_value)
{
u16 brightness = brightness_value + 400;
msg->brightness[0] = brightness & 0xff;
msg->brightness[2] = brightness >> 8;
}

This is similar to what is done in drm_mipi_dsi.
Other backlight drivers (except one) uses similar tricks to handle when
the brightness is more than one byte.

The magic number 400 would be better represented by a constant like:
#define BACKLIGHT_INTENSITY_OFFSET 400
Or something like this.

It also from the code looks like unknown_1 is a command byte.
#define GET_BACKLIGHT_INTENSITY 0x0
#define SET_BACKLIGHT_INTENSITY 0x1

Looks more descriptive than the current hard coding, implicit via memset
or explicit.

> +void init_ctrl_msg_data(struct brightness_ctrl_message_data *msg)
> +{
> + memset(msg, 0, sizeof(struct brightness_ctrl_message_data));
> + msg->unknown_1 = 0x01;
> +}
As the build bot already told you, please use static everywhere
possible.
In this case just drop the helper as it has only one user.

> +
> +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);
The struct is so small that you can safely have it as a local variable.
The pointer is almost the same size. And then there is no need to check
for failing allocations.

> + 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);
Consider specifying the number of msec to wait, rather than the platform
dependent HZ value that may or may not work.
I found:
#define USB_CTRL_GET_TIMEOUT 5000
#define USB_CTRL_SET_TIMEOUT 5000

They look like the right choices here.

> + if (err < 0) {
> + dev_err(&interface->dev,
> + "get: usb control message err: %d\n",
> + err);
> + }
> + *brightness = get_ctrl_message_brightness(msg_data);
Should this check the first byte that I assume is a command byte?

> + 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);
As above, declare it on the stack.

> + 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);
Here you should replace bl->props.brightness with
backlight_get_brightness(bl).
This will give you the value 0 when the backlight device is power down
or blank which is often the right thing.

> + 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;
Use dev_err_probe() here.
> + }
> +
> + if (interface->cur_altsetting->desc.bInterfaceNumber != brightness_interface_nr)
> + return -ENODEV;
Same here, so you also log something.
> +
> + 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);
Same here.
> + }
> + 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);
Any particular reason NOT to use devm_backlight_device_register()?

> + 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@xxxxxxx>");
> +MODULE_LICENSE("Dual MIT/GPL");
> +MODULE_DESCRIPTION("Backlight control for USB-C Thunderbolt Apple displays");
> --
> 2.41.0