backlight: apple_bl: Use acpi_video_get_backlight_type()

On some MacBooks both the apple_bl and the apple-gmux backlight drivers
may be able to export a /sys/class/backlight device.

To avoid having 2 backlight devices for one LCD panel until now
the apple-gmux driver has been calling apple_bl_unregister() to move
the apple_bl backlight device out of the way when it loads.

Similar problems exist on other x86 laptops and all backlight drivers
which may be used on x86 laptops have moved to using
acpi_video_get_backlight_type() to determine whether they should load
or not.

Switch apple_bl to this model too, so that it is consistent with all
the other x86 backlight drivers.

Besides code-simplification and consistency this has 2 other benefits:

1) It removes a race during boot where userspace will briefly see
an apple_bl backlight and then have it disappear again, leading to e.g.:
https://bbs.archlinux.org/viewtopic.php?id=269920

2) This allows user to switch between the drivers by passing
acpi_backlight=apple_gmux or acpi_backlight=vendor on the kernel
commandline.

Signed-off-by: Hans de Goede <[e-mail]>
Reviewed-by: Daniel Thompson <[e-mail]>
Acked-by: Lee Jones <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/platform/x86/Kconfig

···
 	depends on ACPI && PCI
 	depends on PNP
 	depends on BACKLIGHT_CLASS_DEVICE
-	depends on BACKLIGHT_APPLE=n || BACKLIGHT_APPLE
 	help
 	  This driver provides support for the gmux device found on many
 	  Apple laptops, which controls the display mux for the hybrid

drivers/platform/x86/apple-gmux.c

···
 #include <linux/backlight.h>
 #include <linux/acpi.h>
 #include <linux/pnp.h>
-#include <linux/apple_bl.h>
 #include <linux/apple-gmux.h>
 #include <linux/slab.h>
 #include <linux/delay.h>
···
 		gmux_data->bdev = bdev;
 		bdev->props.brightness = gmux_get_brightness(bdev);
 		backlight_update_status(bdev);
-
-		/*
-		 * The backlight situation on Macs is complicated. If the gmux is
-		 * present it's the best choice, because it always works for
-		 * backlight control and supports more levels than other options.
-		 * Disable the other backlight choices.
-		 */
-		apple_bl_unregister();
 	}
 
 	gmux_data->power_state = VGA_SWITCHEROO_ON;
···
 		release_region(gmux_data->iostart, gmux_data->iolen);
 	apple_gmux_data = NULL;
 	kfree(gmux_data);
-
-	apple_bl_register();
 }
 
 static const struct pnp_device_id gmux_device_ids[] = {

drivers/video/backlight/Kconfig

···
 config BACKLIGHT_APPLE
 	tristate "Apple Backlight Driver"
 	depends on X86 && ACPI
+	depends on ACPI_VIDEO=n || ACPI_VIDEO
 	help
 	  If you have an Intel-based Apple say Y to enable a driver for its
 	  backlight.

drivers/video/backlight/apple_bl.c

···
 #include <linux/pci.h>
 #include <linux/acpi.h>
 #include <linux/atomic.h>
-#include <linux/apple_bl.h>
+#include <acpi/video.h>
 
 static struct backlight_device *apple_backlight_device;
 
···
 	},
 };
 
-static atomic_t apple_bl_registered = ATOMIC_INIT(0);
-
-int apple_bl_register(void)
-{
-	if (atomic_xchg(&apple_bl_registered, 1) == 0)
-		return acpi_bus_register_driver(&apple_bl_driver);
-
-	return 0;
-}
-EXPORT_SYMBOL_GPL(apple_bl_register);
-
-void apple_bl_unregister(void)
-{
-	if (atomic_xchg(&apple_bl_registered, 0) == 1)
-		acpi_bus_unregister_driver(&apple_bl_driver);
-}
-EXPORT_SYMBOL_GPL(apple_bl_unregister);
-
 static int __init apple_bl_init(void)
 {
-	return apple_bl_register();
+	/*
+	 * Use ACPI video detection code to see if this driver should register
+	 * or if another driver, e.g. the apple-gmux driver should be used.
+	 */
+	if (acpi_video_get_backlight_type() != acpi_backlight_vendor)
+		return -ENODEV;
+
+	return acpi_bus_register_driver(&apple_bl_driver);
 }
 
 static void __exit apple_bl_exit(void)
 {
-	apple_bl_unregister();
+	acpi_bus_unregister_driver(&apple_bl_driver);
 }
 
 module_init(apple_bl_init);

include/linux/apple_bl.h

···
-/* SPDX-License-Identifier: GPL-2.0 */
-/*
- * apple_bl exported symbols
- */
-
-#ifndef _LINUX_APPLE_BL_H
-#define _LINUX_APPLE_BL_H
-
-#if defined(CONFIG_BACKLIGHT_APPLE) || defined(CONFIG_BACKLIGHT_APPLE_MODULE)
-
-extern int apple_bl_register(void);
-extern void apple_bl_unregister(void);
-
-#else /* !CONFIG_BACKLIGHT_APPLE */
-
-static inline int apple_bl_register(void)
-{
-	return 0;
-}
-
-static inline void apple_bl_unregister(void)
-{
-}
-
-#endif /* !CONFIG_BACKLIGHT_APPLE */
-
-#endif /* _LINUX_APPLE_BL_H */