linux-kernel - Re: [PATCH v2] ACPI: video: Put ACPI video and its child devices to D0 at boot

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Message-ID: <CAAd53p5+3GOu9cCob2FiYEtSVCHEsJQi72PDkotK7FniwVpETg@mail.gmail.com>
Date:   Fri, 21 Jul 2023 23:05:51 +0800
From:   Kai-Heng Feng <kai.heng.feng@...onical.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     lenb@...nel.org, hdegoede@...hat.com, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ACPI: video: Put ACPI video and its child devices to
 D0 at boot

On Fri, Jul 21, 2023 at 1:32 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Wed, Jul 19, 2023 at 4:33 AM Kai-Heng Feng
> <kai.heng.feng@...onical.com> wrote:
> >
> > Screen brightness can only be changed once on some HP laptops.
> >
> > Vendor identified the root cause as Linux doesn't invoke _PS0 at boot
> > for all ACPI devices:
>
> No, it doesn't and it won't.  For the reason why please see the
> comment in acpi_bus_init_power().

Understood.

>
> BTW, missing _PSC for a device that has _PS0 violates Section 7.3.6.
> _PSC (Power State Current) of the ACPI specification, so this is a
> platform firmware problem.

This means most if not all platforms have this problem.

>
> >     Scope (\_SB.PC00.GFX0)
> >     {
> >         Scope (DD1F)
> >         {
> >             Method (_PS0, 0, Serialized)  // _PS0: Power State 0
> >             {
> >                 If (CondRefOf (\_SB.PC00.LPCB.EC0.SSBC))
> >                 {
> >                     \_SB.PC00.LPCB.EC0.SSBC ()
> >                 }
> >             }
> >             ...
> >         }
> >         ...
> >     }
> >
> > The \_SB.PC00.GFX0.DD1F is the panel device, and its _PS0 needs to be
> > executed to make the brightness control work properly.
> >
> > _PS0 doesn't get invoked for all ACPI devices because of commit
> > 7cd8407d53ef ("ACPI / PM: Do not execute _PS0 for devices without _PSC
> > during initialization"). For now use acpi_device_fix_up_power_extended()
> > to put ACPI video and its child devices to D0 to workaround the issue.
>
> The above paragraph is misleading the reader into thinking that the
> issue was introduced by a kernel change while the issue really is
> there in the platform firmware and it is the missing _PSC.
>
> Blaming kernel changes for platform firmware defects is not really productive.

Right, I'll reword the message and make it clear that it's firmware's issue.

>
> So this really is a workaround for defective platform firmware on the
> affected systems and it should be documented as such.

Sure.

>
> > Link: https://bugzilla.kernel.org/show_bug.cgi?id=217683
> > Signed-off-by: Kai-Heng Feng <kai.heng.feng@...onical.com>
> > ---
> > v2:
> >  - Wording
> >  - Bugzilla
> >  - Add comment
> >
> >  drivers/acpi/acpi_video.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/drivers/acpi/acpi_video.c b/drivers/acpi/acpi_video.c
> > index 62f4364e4460..bac614e9fe56 100644
> > --- a/drivers/acpi/acpi_video.c
> > +++ b/drivers/acpi/acpi_video.c
> > @@ -2027,6 +2027,13 @@ static int acpi_video_bus_add(struct acpi_device *device)
> >         if (error)
> >                 goto err_put_video;
> >
> > +
> > +       /*
> > +        * Some HP laptops require ACPI video's child devices have _PS0
> > +        * evaluated to have functional panel brightness control.
>
> So the comment should mention the platform firmware defect and it
> shouldn't say "some" but explicitly identify the defective platforms.

OK.

>
> > +        */
> > +       acpi_device_fix_up_power_extended(device);
> > +
>
> And the code change itself is simple enough, so it can be made as long
> as it is fine with Hans.

OK.


Kai-Heng

>
> >         pr_info("%s [%s] (multi-head: %s  rom: %s  post: %s)\n",
> >                ACPI_VIDEO_DEVICE_NAME, acpi_device_bid(device),
> >                video->flags.multihead ? "yes" : "no",
> > --