[RFC 2/2] ACPI: x86: Add DELL0501 handling to acpi_quirk_skip_serdev_enumeration()

[Hans de Goede <hdegoede@redhat.com>]

Some recent(ish) Dell AIO devices have a backlight controller board
connected to an UART.

This UART has a DELL0501 HID with CID set to PNP0501 so that the UART is
still handled by 8250_pnp.c. Unfortunately there is no separate ACPI device
with an UartSerialBusV2() resource to model the backlight-controller.
This causes the kernel to create a /dev/ttyS0 char-device for the UART
instead of creating an in kernel serdev-controller + serdev-device pair
for a kernel backlight driver.

Use the existing acpi_quirk_skip_serdev_enumeration() mechanism to work
around this by returning skip=true for tty-ctrl parents with a HID
of DELL0501.

Like other cases where the UartSerialBusV2() resource is missing or broken
this will only create the serdev-controller device and the serdev-device
itself will need to be instantiated by platform code.

Unfortunately in this case there is no device for the platform-code
instantiating the serdev-device to bind to. So also create
a platform_device for this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
---
 drivers/acpi/x86/utils.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/drivers/acpi/x86/utils.c b/drivers/acpi/x86/utils.c
index 185176a521ad..c7af2d2986fd 100644
--- a/drivers/acpi/x86/utils.c
+++ b/drivers/acpi/x86/utils.c
@@ -493,8 +493,28 @@ static int acpi_dmi_skip_serdev_enumeration(struct device *controller_parent, bo
 
 int acpi_quirk_skip_serdev_enumeration(struct device *controller_parent, bool *skip)
 {
+	struct acpi_device *adev = ACPI_COMPANION(controller_parent);
+
 	*skip = false;
 
+	/*
+	 * The DELL0501 ACPI HID represents an UART (CID is set to PNP0501) with
+	 * a backlight-controller attached. There is no separate ACPI device with
+	 * an UartSerialBusV2() resource to model the backlight-controller.
+	 * Set skip to true so that the tty core creates a serdev ctrl device.
+	 * The backlight driver will manually create the serdev client device.
+	 */
+	if (acpi_dev_hid_match(adev, "DELL0501")) {
+		*skip = true;
+		/*
+		 * Create a platform dev for dell-uart-backlight to bind to.
+		 * This is a static device, so no need to store the result.
+		 */
+		platform_device_register_simple("dell-uart-backlight", PLATFORM_DEVID_NONE,
+						NULL, 0);
+		return 0;
+	}
+
 	return acpi_dmi_skip_serdev_enumeration(controller_parent, skip);
 }
 EXPORT_SYMBOL_GPL(acpi_quirk_skip_serdev_enumeration);
-- 
2.43.0