[Intel-gfx] [PATCH] ACPI: Fix selecting the wrong ACPI fwnode for the iGPU on some Dell laptops

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

The Dell Latitude E6430 both with and without the optional NVidia dGPU
has a bug in its ACPI tables which is causing Linux to assign the wrong
ACPI fwnode / companion to the pci_device for the i915 iGPU.

Specifically under the PCI root bridge there are these 2 ACPI Device()s :

 Scope (_SB.PCI0)
 {
     Device (GFX0)
     {
         Name (_ADR, 0x00020000)  // _ADR: Address
     }

     ...

     Device (VID)
     {
         Name (_ADR, 0x00020000)  // _ADR: Address
         ...

         Method (_DOS, 1, NotSerialized)  // _DOS: Disable Output Switching
         {
             VDP8 = Arg0
             VDP1 (One, VDP8)
         }

         Method (_DOD, 0, NotSerialized)  // _DOD: Display Output Devices
         {
             ...
         }
         ...
     }
 }

The non-functional GFX0 ACPI device is a problem, because this gets
returned as ACPI companion-device by acpi_find_child_device() for the iGPU.

This is a long standing problem and the i915 driver does use the ACPI
companion for some things, but works fine without it.

However since commit 63f534b8bad9 ("ACPI: PCI: Rework acpi_get_pci_dev()")
acpi_get_pci_dev() relies on the physical-node pointer in the acpi_device
and that is set on the wrong acpi_device because of the wrong
acpi_find_child_device() return. This breaks the ACPI video code, leading
to non working backlight control in some cases.

Make find_child_checks() return a higher score for children which have
pnp-ids set by various scan helpers like acpi_is_video_device(), so
that it picks the right companion-device.

An alternative approach would be to directly call acpi_is_video_device()
from find_child_checks() but that would be somewhat computationally
expensive given that acpi_find_child_device() iterates over all the
PCI0 children every time it is called.

Fixes: 63f534b8bad9 ("ACPI: PCI: Rework acpi_get_pci_dev()")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
---
 drivers/acpi/glue.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/acpi/glue.c b/drivers/acpi/glue.c
index 204fe94c7e45..2055dfd7678b 100644
--- a/drivers/acpi/glue.c
+++ b/drivers/acpi/glue.c
@@ -75,7 +75,7 @@ static struct acpi_bus_type *acpi_get_bus_type(struct device *dev)
 }
 
 #define FIND_CHILD_MIN_SCORE	1
-#define FIND_CHILD_MAX_SCORE	2
+#define FIND_CHILD_MAX_SCORE	3
 
 static int match_any(struct acpi_device *adev, void *not_used)
 {
@@ -89,15 +89,25 @@ static bool acpi_dev_has_children(struct acpi_device *adev)
 
 static int find_child_checks(struct acpi_device *adev, bool check_children)
 {
+	int score = FIND_CHILD_MIN_SCORE;
 	unsigned long long sta;
 	acpi_status status;
 
 	if (check_children && !acpi_dev_has_children(adev))
 		return -ENODEV;
 
+	/*
+	 * For devices without a _STA method, prefer devices without a _HID
+	 * (which conflicts with having an _ADR) but which have been matched
+	 * in some other way, like e.g. by acpi_is_video_device() over devices
+	 * with no ids at all.
+	 */
+	if (!adev->pnp.type.platform_id && adev->pnp.type.hardware_id)
+		score++;
+
 	status = acpi_evaluate_integer(adev->handle, "_STA", NULL, &sta);
 	if (status == AE_NOT_FOUND)
-		return FIND_CHILD_MIN_SCORE;
+		return score;
 
 	if (ACPI_FAILURE(status) || !(sta & ACPI_STA_DEVICE_ENABLED))
 		return -ENODEV;
-- 
2.39.0