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

[Hans de Goede]

Hi Rafael,

On 1/10/23 14:33, Rafael J. Wysocki wrote:
> On Monday, January 9, 2023 9:57:21 PM CET Hans de Goede wrote:
>> 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.
> 
> Interesting.  Sorry for the trouble.

No problem, as mentioned this is actually a long standing issue / bug
in the ACPI tables, it just never surfaced before.

>> 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.
> 
> This has a potential of changing the behavior in some cases that are not
> relevant here which is generally risky.
> 
>> 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.
> 
> I agree with the above, but my fix would be something like the patch below (not
> really tested, but it builds).

Thanks, I have just given this a spin on my E6430 and I can confirm
it still fixes things.

I'll send out this version (re-using most of the v1 commitmsg) as a v2
right away.

Regards,

Hans





> 
> ---
>  drivers/acpi/glue.c     |   14 ++++++++++++--
>  drivers/acpi/scan.c     |    7 +++++--
>  include/acpi/acpi_bus.h |    3 ++-
>  3 files changed, 19 insertions(+), 5 deletions(-)
> 
> Index: linux-pm/include/acpi/acpi_bus.h
> ===================================================================
> --- linux-pm.orig/include/acpi/acpi_bus.h
> +++ linux-pm/include/acpi/acpi_bus.h
> @@ -230,7 +230,8 @@ struct acpi_pnp_type {
>  	u32 hardware_id:1;
>  	u32 bus_address:1;
>  	u32 platform_id:1;
> -	u32 reserved:29;
> +	u32 backlight:1;
> +	u32 reserved:28;
>  };
>  
>  struct acpi_device_pnp {
> Index: linux-pm/drivers/acpi/scan.c
> ===================================================================
> --- linux-pm.orig/drivers/acpi/scan.c
> +++ linux-pm/drivers/acpi/scan.c
> @@ -1370,9 +1370,12 @@ static void acpi_set_pnp_ids(acpi_handle
>  		 * Some devices don't reliably have _HIDs & _CIDs, so add
>  		 * synthetic HIDs to make sure drivers can find them.
>  		 */
> -		if (acpi_is_video_device(handle))
> +		if (acpi_is_video_device(handle)) {
>  			acpi_add_id(pnp, ACPI_VIDEO_HID);
> -		else if (acpi_bay_match(handle))
> +			pnp->type.backlight = 1;
> +			break;
> +		}
> +		if (acpi_bay_match(handle))
>  			acpi_add_id(pnp, ACPI_BAY_HID);
>  		else if (acpi_dock_match(handle))
>  			acpi_add_id(pnp, ACPI_DOCK_HID);
> Index: linux-pm/drivers/acpi/glue.c
> ===================================================================
> --- linux-pm.orig/drivers/acpi/glue.c
> +++ linux-pm/drivers/acpi/glue.c
> @@ -75,7 +75,8 @@ static struct acpi_bus_type *acpi_get_bu
>  }
>  
>  #define FIND_CHILD_MIN_SCORE	1
> -#define FIND_CHILD_MAX_SCORE	2
> +#define FIND_CHILD_MID_SCORE	2
> +#define FIND_CHILD_MAX_SCORE	3
>  
>  static int match_any(struct acpi_device *adev, void *not_used)
>  {
> @@ -96,8 +97,17 @@ static int find_child_checks(struct acpi
>  		return -ENODEV;
>  
>  	status = acpi_evaluate_integer(adev->handle, "_STA", NULL, &sta);
> -	if (status == AE_NOT_FOUND)
> +	if (status == AE_NOT_FOUND) {
> +		/*
> +		 * Special case: backlight device objects without _STA are
> +		 * preferred to other objects with the same _ADR value, because
> +		 * it is more likely that they are actually useful.
> +		 */
> +		if (adev->pnp.type.backlight)
> +			return FIND_CHILD_MID_SCORE;
> +
>  		return FIND_CHILD_MIN_SCORE;
> +	}
>  
>  	if (ACPI_FAILURE(status) || !(sta & ACPI_STA_DEVICE_ENABLED))
>  		return -ENODEV;
> 
> 
>