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