[PATCH] ACPI: video: Fix Apple GMUX backlight detection

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

The apple-gmux driver only binds to old GMUX devices which have an
IORESOURCE_IO resource (using inb()/outb()) rather then memory-mapped
IO (IORESOURCE_MEM).

T2 MacBooks use the new style GMUX devices (with IORESOURCE_MEM access),
so these are not supported by the apple-gmux driver. This is not a problem
since they have working ACPI video backlight support.

But the apple_gmux_present() helper only checks if an ACPI device with
the "APP000B" HID is present, causing acpi_video_get_backlight_type()
to return acpi_backlight_apple_gmux disabling the acpi_video backlight
device.

Add a new apple_gmux_backlight_present() helper which checks that
the "APP000B" device actually is an old GMUX device with an IORESOURCE_IO
resource.

This fixes the acpi_video0 backlight no longer registering on T2 MacBooks.

Note people are working to add support for the new style GMUX to Linux:
https://github.com/kekrby/linux-t2/commits/wip/hybrid-graphics

Once this lands this patch should be reverted so that
acpi_video_get_backlight_type() also prefers the gmux on new style GMUX
MacBooks, but for now this is necessary to avoid regressing backlight
control on T2 Macs.

Fixes: 21245df307cb ("ACPI: video: Add Apple GMUX brightness control detection")
Reported-and-tested-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
---
 drivers/acpi/video_detect.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/drivers/acpi/video_detect.c b/drivers/acpi/video_detect.c
index a934bbc9dd37..1b78c7434492 100644
--- a/drivers/acpi/video_detect.c
+++ b/drivers/acpi/video_detect.c
@@ -34,6 +34,7 @@
 #include <linux/module.h>
 #include <linux/pci.h>
 #include <linux/platform_data/x86/nvidia-wmi-ec-backlight.h>
+#include <linux/pnp.h>
 #include <linux/types.h>
 #include <linux/workqueue.h>
 #include <acpi/video.h>
@@ -105,6 +106,26 @@ static bool nvidia_wmi_ec_supported(void)
 }
 #endif
 
+static bool apple_gmux_backlight_present(void)
+{
+	struct acpi_device *adev;
+	struct device *dev;
+
+	adev = acpi_dev_get_first_match_dev(GMUX_ACPI_HID, NULL, -1);
+	if (!adev)
+		return false;
+
+	dev = acpi_get_first_physical_node(adev);
+	if (!dev)
+		return false;
+
+	/*
+	 * drivers/platform/x86/apple-gmux.c only supports old style
+	 * Apple GMUX with an IO-resource.
+	 */
+	return pnp_get_resource(to_pnp_dev(dev), IORESOURCE_IO, 0) != NULL;
+}
+
 /* Force to use vendor driver when the ACPI device is known to be
  * buggy */
 static int video_detect_force_vendor(const struct dmi_system_id *d)
@@ -767,7 +788,7 @@ static enum acpi_backlight_type __acpi_video_get_backlight_type(bool native)
 	if (nvidia_wmi_ec_present)
 		return acpi_backlight_nvidia_wmi_ec;
 
-	if (apple_gmux_present())
+	if (apple_gmux_backlight_present())
 		return acpi_backlight_apple_gmux;
 
 	/* Use ACPI video if available, except when native should be preferred. */
-- 
2.38.1