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Date: Wed, 12 Jun 2024 18:33:20 +0000
From: Sakari Ailus <sakari.ailus@linux.intel.com>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Hans de Goede <hdegoede@redhat.com>, Genes Lists <lists@sapience.com>,
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	"regressions@lists.linux.dev" <regressions@lists.linux.dev>
Subject: Re: [PATCH 1/1] ACPI: scan: Ignore Dell XPS 9320 camera graph port
 nodes
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Hi Rafael,

On Wed, Jun 12, 2024 at 05:26:46PM +0200, Rafael J. Wysocki wrote:
> Hi,
> 
> On Wed, Jun 12, 2024 at 4:30 PM Hans de Goede <hdegoede@redhat.com> wrote:
> >
> > 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.
> 
> Well, if Windows doesn't use this information, it is almost guaranteed
> to be garbage.

No ACPI DSDT in production systems uses DisCo for Imaging as of now at
least to my knowledge.

> 
> So maybe it would be better to make acpi_graph_ignore_port() return
> true by default and false only when the information is known to be
> valid.  IOW, whitelist things instead of adding blacklist entries in
> perpetuum.

What could be gained from this?

> 
> And hopefully we'll eventually get to the point at which we are able
> to say "whitelist everything from now on".

-- 
Kind regards,

Sakari Ailus