platform/x86: Add new Dell UART backlight driver

Dell All In One (AIO) models released after 2017 use a backlight controller
board connected to an UART.

In DSDT this uart port will be defined as:

   Name (_HID, "DELL0501")
   Name (_CID, EisaId ("PNP0501")

Instead of having a separate ACPI device with an UartSerialBusV2() resource
to model the backlight-controller, which would be the standard way to do
this.

The acpi_quirk_skip_serdev_enumeration() has special handling for this
and it will make the serial port code create a serdev controller device
for the UART instead of a /dev/ttyS0 char-dev. It will also create
a dell-uart-backlight driver platform device for this driver to bind too.

This new kernel module contains 2 drivers for this:

1. A simple platform driver which creates the actual serdev device
   (with the serdev controller device as parent)

2. A serdev driver for the created serdev device which exports
   the backlight functionality uses a standard backlight class device.

Reported-by: Roman Bogoyev <roman@computercheck.com.au>
Tested-by: Roman Bogoyev <roman@computercheck.com.au>
Tested-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Co-developed-by: AceLan Kao <acelan.kao@canonical.com>
Signed-off-by: AceLan Kao <acelan.kao@canonical.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20240513144603.93874-2-hdegoede@redhat.com

drivers/platform/x86/dell/Kconfig

@@ -145,6 +145,21 @@ config DELL_SMO8800
 	  To compile this driver as a module, choose M here: the module will
 	  be called dell-smo8800.
 
+config DELL_UART_BACKLIGHT
+	tristate "Dell AIO UART Backlight driver"
+	depends on ACPI
+	depends on BACKLIGHT_CLASS_DEVICE
+	depends on SERIAL_DEV_BUS
+	help
+	  Say Y here if you want to support Dell AIO UART backlight interface.
+	  The Dell AIO machines released after 2017 come with a UART interface
+	  to communicate with the backlight scalar board. This driver creates
+	  a standard backlight interface and talks to the scalar board through
+	  UART to adjust the AIO screen brightness.
+
+	  To compile this driver as a module, choose M here: the module will
+	  be called dell_uart_backlight.
+
 config DELL_WMI
 	tristate "Dell WMI notifications"
 	default m

drivers/platform/x86/dell/Makefile

@@ -14,6 +14,7 @@ dell-smbios-objs			:= dell-smbios-base.o
 dell-smbios-$(CONFIG_DELL_SMBIOS_WMI)	+= dell-smbios-wmi.o
 dell-smbios-$(CONFIG_DELL_SMBIOS_SMM)	+= dell-smbios-smm.o
 obj-$(CONFIG_DELL_SMO8800)		+= dell-smo8800.o
+obj-$(CONFIG_DELL_UART_BACKLIGHT)	+= dell-uart-backlight.o
 obj-$(CONFIG_DELL_WMI)			+= dell-wmi.o
 dell-wmi-objs				:= dell-wmi-base.o
 dell-wmi-$(CONFIG_DELL_WMI_PRIVACY)	+= dell-wmi-privacy.o

drivers/platform/x86/dell/dell-uart-backlight.c

@@ -0,0 +1,398 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Dell AIO Serial Backlight Driver
+ *
+ * Copyright (C) 2024 Hans de Goede <hansg@kernel.org>
+ * Copyright (C) 2017 AceLan Kao <acelan.kao@canonical.com>
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/acpi.h>
+#include <linux/backlight.h>
+#include <linux/delay.h>
+#include <linux/device.h>
+#include <linux/err.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/platform_device.h>
+#include <linux/serdev.h>
+#include <linux/string.h>
+#include <linux/types.h>
+#include <linux/wait.h>
+#include "../serdev_helpers.h"
+
+/* The backlight controller must respond within 1 second */
+#define DELL_BL_TIMEOUT		msecs_to_jiffies(1000)
+#define DELL_BL_MAX_BRIGHTNESS	100
+
+/* Defines for the commands send to the controller */
+
+/* 1st byte Start Of Frame 3 MSB bits: cmd-len + 01010 SOF marker */
+#define DELL_SOF(len)			(((len) << 5) | 0x0a)
+#define GET_CMD_LEN			3
+#define SET_CMD_LEN			4
+
+/* 2nd byte command */
+#define CMD_GET_VERSION			0x06
+#define CMD_SET_BRIGHTNESS		0x0b
+#define CMD_GET_BRIGHTNESS		0x0c
+#define CMD_SET_BL_POWER		0x0e
+
+/* Indexes and other defines for response received from the controller */
+#define RESP_LEN			0
+#define RESP_CMD			1 /* Echo of CMD byte from command */
+#define RESP_DATA			2 /* Start of received data */
+
+#define SET_RESP_LEN			3
+#define GET_RESP_LEN			4
+#define MIN_RESP_LEN			3
+#define MAX_RESP_LEN			80
+
+struct dell_uart_backlight {
+	struct mutex mutex;
+	wait_queue_head_t wait_queue;
+	struct device *dev;
+	struct backlight_device *bl;
+	u8 *resp;
+	u8 resp_idx;
+	u8 resp_len;
+	u8 resp_max_len;
+	u8 pending_cmd;
+	int status;
+	int power;
+};
+
+/* Checksum: SUM(Length and Cmd and Data) xor 0xFF */
+static u8 dell_uart_checksum(u8 *buf, int len)
+{
+	u8 val = 0;
+
+	while (len-- > 0)
+		val += buf[len];
+
+	return val ^ 0xff;
+}
+
+static int dell_uart_bl_command(struct dell_uart_backlight *dell_bl,
+				const u8 *cmd, int cmd_len,
+				u8 *resp, int resp_max_len)
+{
+	int ret;
+
+	ret = mutex_lock_killable(&dell_bl->mutex);
+	if (ret)
+		return ret;
+
+	dell_bl->status = -EBUSY;
+	dell_bl->resp = resp;
+	dell_bl->resp_idx = 0;
+	dell_bl->resp_len = -1; /* Invalid / unset */
+	dell_bl->resp_max_len = resp_max_len;
+	dell_bl->pending_cmd = cmd[1];
+
+	/* The TTY buffer should be big enough to take the entire cmd in one go */
+	ret = serdev_device_write_buf(to_serdev_device(dell_bl->dev), cmd, cmd_len);
+	if (ret != cmd_len) {
+		dev_err(dell_bl->dev, "Error writing command: %d\n", ret);
+		dell_bl->status = (ret < 0) ? ret : -EIO;
+		goto out;
+	}
+
+	ret = wait_event_timeout(dell_bl->wait_queue, dell_bl->status != -EBUSY,
+				 DELL_BL_TIMEOUT);
+	if (ret == 0) {
+		dev_err(dell_bl->dev, "Timed out waiting for response.\n");
+		/* Clear busy status to discard bytes received after this */
+		dell_bl->status = -ETIMEDOUT;
+	}
+
+out:
+	mutex_unlock(&dell_bl->mutex);
+	return dell_bl->status;
+}
+
+static int dell_uart_set_brightness(struct dell_uart_backlight *dell_bl, int brightness)
+{
+	u8 set_brightness[SET_CMD_LEN], resp[SET_RESP_LEN];
+
+	set_brightness[0] = DELL_SOF(SET_CMD_LEN);
+	set_brightness[1] = CMD_SET_BRIGHTNESS;
+	set_brightness[2] = brightness;
+	set_brightness[3] = dell_uart_checksum(set_brightness, 3);
+
+	return dell_uart_bl_command(dell_bl, set_brightness, SET_CMD_LEN, resp, SET_RESP_LEN);
+}
+
+static int dell_uart_get_brightness(struct dell_uart_backlight *dell_bl)
+{
+	struct device *dev = dell_bl->dev;
+	u8 get_brightness[GET_CMD_LEN], resp[GET_RESP_LEN];
+	int ret;
+
+	get_brightness[0] = DELL_SOF(GET_CMD_LEN);
+	get_brightness[1] = CMD_GET_BRIGHTNESS;
+	get_brightness[2] = dell_uart_checksum(get_brightness, 2);
+
+	ret = dell_uart_bl_command(dell_bl, get_brightness, GET_CMD_LEN, resp, GET_RESP_LEN);
+	if (ret)
+		return ret;
+
+	if (resp[RESP_LEN] != GET_RESP_LEN) {
+		dev_err(dev, "Unexpected get brightness response length: %d\n", resp[RESP_LEN]);
+		return -EIO;
+	}
+
+	if (resp[RESP_DATA] > DELL_BL_MAX_BRIGHTNESS) {
+		dev_err(dev, "Unexpected get brightness response: %d\n", resp[RESP_DATA]);
+		return -EIO;
+	}
+
+	return resp[RESP_DATA];
+}
+
+static int dell_uart_set_bl_power(struct dell_uart_backlight *dell_bl, int power)
+{
+	u8 set_power[SET_CMD_LEN], resp[SET_RESP_LEN];
+	int ret;
+
+	set_power[0] = DELL_SOF(SET_CMD_LEN);
+	set_power[1] = CMD_SET_BL_POWER;
+	set_power[2] = (power == FB_BLANK_UNBLANK) ? 1 : 0;
+	set_power[3] = dell_uart_checksum(set_power, 3);
+
+	ret = dell_uart_bl_command(dell_bl, set_power, SET_CMD_LEN, resp, SET_RESP_LEN);
+	if (ret)
+		return ret;
+
+	dell_bl->power = power;
+	return 0;
+}
+
+/*
+ * There is no command to get backlight power status,
+ * so we set the backlight power to "on" while initializing,
+ * and then track and report its status by power variable.
+ */
+static int dell_uart_get_bl_power(struct dell_uart_backlight *dell_bl)
+{
+	return dell_bl->power;
+}
+
+static int dell_uart_update_status(struct backlight_device *bd)
+{
+	struct dell_uart_backlight *dell_bl = bl_get_data(bd);
+	int ret;
+
+	ret = dell_uart_set_brightness(dell_bl, bd->props.brightness);
+	if (ret)
+		return ret;
+
+	if (bd->props.power != dell_uart_get_bl_power(dell_bl))
+		return dell_uart_set_bl_power(dell_bl, bd->props.power);
+
+	return 0;
+}
+
+static int dell_uart_get_brightness_op(struct backlight_device *bd)
+{
+	return dell_uart_get_brightness(bl_get_data(bd));
+}
+
+static const struct backlight_ops dell_uart_backlight_ops = {
+	.update_status = dell_uart_update_status,
+	.get_brightness = dell_uart_get_brightness_op,
+};
+
+static size_t dell_uart_bl_receive(struct serdev_device *serdev, const u8 *data, size_t len)
+{
+	struct dell_uart_backlight *dell_bl = serdev_device_get_drvdata(serdev);
+	size_t i;
+	u8 csum;
+
+	dev_dbg(dell_bl->dev, "Recv: %*ph\n", (int)len, data);
+
+	/* Throw away unexpected bytes / remainder of response after an error */
+	if (dell_bl->status != -EBUSY) {
+		dev_warn(dell_bl->dev, "Bytes received out of band, dropping them.\n");
+		return len;
+	}
+
+	i = 0;
+	while (i < len && dell_bl->resp_idx != dell_bl->resp_len) {
+		dell_bl->resp[dell_bl->resp_idx] = data[i++];
+
+		switch (dell_bl->resp_idx) {
+		case RESP_LEN: /* Length byte */
+			dell_bl->resp_len = dell_bl->resp[RESP_LEN];
+			if (dell_bl->resp_len < MIN_RESP_LEN ||
+			    dell_bl->resp_len > dell_bl->resp_max_len) {
+				dev_err(dell_bl->dev, "Response length %d out if range %d - %d\n",
+					dell_bl->resp_len, MIN_RESP_LEN, dell_bl->resp_max_len);
+				dell_bl->status = -EIO;
+				goto wakeup;
+			}
+			break;
+		case RESP_CMD: /* CMD byte */
+			if (dell_bl->resp[RESP_CMD] != dell_bl->pending_cmd) {
+				dev_err(dell_bl->dev, "Response cmd 0x%02x != pending 0x%02x\n",
+					dell_bl->resp[RESP_CMD], dell_bl->pending_cmd);
+				dell_bl->status = -EIO;
+				goto wakeup;
+			}
+			break;
+		}
+		dell_bl->resp_idx++;
+	}
+
+	if (dell_bl->resp_idx != dell_bl->resp_len)
+		return len; /* Response not complete yet */
+
+	csum = dell_uart_checksum(dell_bl->resp, dell_bl->resp_len - 1);
+	if (dell_bl->resp[dell_bl->resp_len - 1] == csum) {
+		dell_bl->status = 0; /* Success */
+	} else {
+		dev_err(dell_bl->dev, "Checksum mismatch got 0x%02x expected 0x%02x\n",
+			dell_bl->resp[dell_bl->resp_len - 1], csum);
+		dell_bl->status = -EIO;
+	}
+wakeup:
+	wake_up(&dell_bl->wait_queue);
+	return i;
+}
+
+static const struct serdev_device_ops dell_uart_bl_serdev_ops = {
+	.receive_buf = dell_uart_bl_receive,
+	.write_wakeup = serdev_device_write_wakeup,
+};
+
+static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
+{
+	u8 get_version[GET_CMD_LEN], resp[MAX_RESP_LEN];
+	struct backlight_properties props = {};
+	struct dell_uart_backlight *dell_bl;
+	struct device *dev = &serdev->dev;
+	int ret;
+
+	dell_bl = devm_kzalloc(dev, sizeof(*dell_bl), GFP_KERNEL);
+	if (!dell_bl)
+		return -ENOMEM;
+
+	mutex_init(&dell_bl->mutex);
+	init_waitqueue_head(&dell_bl->wait_queue);
+	dell_bl->dev = dev;
+
+	ret = devm_serdev_device_open(dev, serdev);
+	if (ret)
+		return dev_err_probe(dev, ret, "opening UART device\n");
+
+	/* 9600 bps, no flow control, these are the default but set them to be sure */
+	serdev_device_set_baudrate(serdev, 9600);
+	serdev_device_set_flow_control(serdev, false);
+	serdev_device_set_drvdata(serdev, dell_bl);
+	serdev_device_set_client_ops(serdev, &dell_uart_bl_serdev_ops);
+
+	get_version[0] = DELL_SOF(GET_CMD_LEN);
+	get_version[1] = CMD_GET_VERSION;
+	get_version[2] = dell_uart_checksum(get_version, 2);
+
+	ret = dell_uart_bl_command(dell_bl, get_version, GET_CMD_LEN, resp, MAX_RESP_LEN);
+	if (ret)
+		return dev_err_probe(dev, ret, "getting firmware version\n");
+
+	dev_dbg(dev, "Firmware version: %.*s\n", resp[RESP_LEN] - 3, resp + RESP_DATA);
+
+	/* Initialize bl_power to a known value */
+	ret = dell_uart_set_bl_power(dell_bl, FB_BLANK_UNBLANK);
+	if (ret)
+		return ret;
+
+	ret = dell_uart_get_brightness(dell_bl);
+	if (ret < 0)
+		return ret;
+
+	props.type = BACKLIGHT_PLATFORM;
+	props.brightness = ret;
+	props.max_brightness = DELL_BL_MAX_BRIGHTNESS;
+	props.power = dell_bl->power;
+
+	dell_bl->bl = devm_backlight_device_register(dev, "dell_uart_backlight",
+						     dev, dell_bl,
+						     &dell_uart_backlight_ops,
+						     &props);
+	return PTR_ERR_OR_ZERO(dell_bl->bl);
+}
+
+struct serdev_device_driver dell_uart_bl_serdev_driver = {
+	.probe = dell_uart_bl_serdev_probe,
+	.driver = {
+		.name = KBUILD_MODNAME,
+	},
+};
+
+static int dell_uart_bl_pdev_probe(struct platform_device *pdev)
+{
+	struct serdev_device *serdev;
+	struct device *ctrl_dev;
+	int ret;
+
+	ctrl_dev = get_serdev_controller("DELL0501", NULL, 0, "serial0");
+	if (IS_ERR(ctrl_dev))
+		return PTR_ERR(ctrl_dev);
+
+	serdev = serdev_device_alloc(to_serdev_controller(ctrl_dev));
+	put_device(ctrl_dev);
+	if (!serdev)
+		return -ENOMEM;
+
+	ret = serdev_device_add(serdev);
+	if (ret) {
+		dev_err(&pdev->dev, "error %d adding serdev\n", ret);
+		serdev_device_put(serdev);
+		return ret;
+	}
+
+	ret = serdev_device_driver_register(&dell_uart_bl_serdev_driver);
+	if (ret)
+		goto err_remove_serdev;
+
+	/*
+	 * serdev device <-> driver matching relies on OF or ACPI matches and
+	 * neither is available here, manually bind the driver.
+	 */
+	ret = device_driver_attach(&dell_uart_bl_serdev_driver.driver, &serdev->dev);
+	if (ret)
+		goto err_unregister_serdev_driver;
+
+	/* So that dell_uart_bl_pdev_remove() can remove the serdev */
+	platform_set_drvdata(pdev, serdev);
+	return 0;
+
+err_unregister_serdev_driver:
+	serdev_device_driver_unregister(&dell_uart_bl_serdev_driver);
+err_remove_serdev:
+	serdev_device_remove(serdev);
+	return ret;
+}
+
+static void dell_uart_bl_pdev_remove(struct platform_device *pdev)
+{
+	struct serdev_device *serdev = platform_get_drvdata(pdev);
+
+	serdev_device_driver_unregister(&dell_uart_bl_serdev_driver);
+	serdev_device_remove(serdev);
+}
+
+static struct platform_driver dell_uart_bl_pdev_driver = {
+	.probe = dell_uart_bl_pdev_probe,
+	.remove_new = dell_uart_bl_pdev_remove,
+	.driver = {
+		.name = "dell-uart-backlight",
+	},
+};
+module_platform_driver(dell_uart_bl_pdev_driver);
+
+MODULE_ALIAS("platform:dell-uart-backlight");
+MODULE_DESCRIPTION("Dell AIO Serial Backlight driver");
+MODULE_AUTHOR("Hans de Goede <hansg@kernel.org>");
+MODULE_LICENSE("GPL");