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Date: Wed, 12 Jun 2024 16:30:30 +0200
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Subject: Re: [PATCH 1/1] ACPI: scan: Ignore Dell XPS 9320 camera graph port
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To: "Rafael J. Wysocki" <rafael@kernel.org>,
 Sakari Ailus <sakari.ailus@linux.intel.com>
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From: Hans de Goede <hdegoede@redhat.com>
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Hi,

On 6/12/24 3:06 PM, Rafael J. Wysocki wrote:
> Hi Sakari,
> 
> On Wed, Jun 12, 2024 at 2:47 PM Sakari Ailus
> <sakari.ailus@linux.intel.com> wrote:
>>
>> Hi Rafael,
>>
>> On Wed, Jun 12, 2024 at 02:32:26PM +0200, Rafael J. Wysocki wrote:
>>>>>>> I just hit the same problem on another Dell laptop. It seems that
>>>>>>> all Dell laptops with IPU6 camera from the Tiger Lake, Alder Lake
>>>>>>> and Raptor Lake generations suffer from this problem.
>>>>>>>
>>>>>>> So instead of playing whack a mole with DMI matches we should
>>>>>>> simply disable ACPI MIPI DISCO support on all Dell laptops
>>>>>>> with those CPUs. I'm preparing a fix for this to replace
>>>>>>> the DMI matching now.
>>>>>>
>>>>>> DisCo for Imaging support shouldn't be dropped on these systems, and this
>>>>>> isn't what your patch does either. Instead the ACPI graph port nodes (as
>>>>>> per Linux specific definitions) are simply dropped, i.e. this isn't related
>>>>>> to DisCo for Imaging at all.
>>>>>
>>>>> So it looks like the changelog of that patch could be improved, right?
>>>>
>>>> Well, yes. The reason the function is in the file is that nearly all camera
>>>> related parsing is located there, not that it would be related to DisCo for
>>>> Imaging as such.
>>>
>>> So IIUC the camera graph port nodes are created by default with the
>>> help of the firmware-supplied information, but if that is defective a
>>> quirk can be added to skip the creation of those ports in which case
>>> they will be created elsewhere.
>>>
>>> Is this correct?
>>
>> Yes.
> 
> So it would be good to add a comment to this effect to
> acpi_nondev_subnode_extract() where acpi_graph_ignore_port() is
> called.
> 
> And there is a somewhat tangential question that occurred to me: If
> the nodes are created elsewhere when acpi_graph_ignore_port() is true,
> why is it necessary to consult the platform firmware for the
> information on them at all?  Wouldn't it be better to simply always
> create them elsewhere?

That is a good question. The ACPI MIPI DISCO specification is an
attempt standardize how MIPI cameras and their sensors are described
in ACPI.

But this is not actually being used by any Windows drivers atm. The windows
drivers rely on their own custom ACPI data which gets translated into
standard Linux device-properties by: drivers/media/pci/intel/ipu-bridge.c 

and so far AFAIK there are 0 laptops where there actually is 100% functional
ACPI MIPI information. I believe that some work is in place to get correct
usable ACPI MIPI information in place in the ACPI tables of some Meteor Lake
laptops. But I believe that there too it does not work yet with the BIOS
version with which current Windows models are shipping. It is being fixed
for systems which have Linux support from the vendor but I suspect that
on other models if ACPI MIPI DISCO information is there it will not
necessarily be reliable because AFAICT Windows does not actually use it.

And TBH this has me worried about camera support for Meteor Lake devices
going forward. We really need to have 1 reliable source of truth here and
using information which is ignored by Windows does not seem like the best
source to use.

Sakari I know you have been pushing for MIPI camera descriptions under
ACPI to move to a standardized format and I can see how that is a good
thing, but atm it seems to mainly cause things to break and before
the ACPI MIPI DISCO support landed in 6.8 we did not have these issues,
since the information used by the ipu-bridge code does seem to be correct.

Regards,

Hans