On Fri, Jul 21, 2023 at 1:32 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
>
> On Wed, Jul 19, 2023 at 4:33 AM Kai-Heng Feng
> <kai.heng.feng@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > ---
> > 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",
> > --
