[PATCH v5 0/4] Fix Win8 backlight issue

[Josh Boyer]

On Fri, Oct 11, 2013 at 4:27 PM, Rafael J. Wysocki <rjw at rjwysocki.net> wrote:
> On Friday, October 11, 2013 06:01:43 PM Josh Boyer wrote:
>> On Fri, Oct 11, 2013 at 6:10 PM, Rafael J. Wysocki <rjw at rjwysocki.net> wrote:
>> > On Friday, October 11, 2013 12:42:43 PM Josh Boyer wrote:
>> >> On Fri, Oct 11, 2013 at 9:27 AM, Aaron Lu <aaron.lu at intel.com> wrote:
>> >> > v5:
>> >> > 1 Introduce video.use_native_backlight module parameter and set its
>> >> >   value to false by default as suggested by Rafael. For Win8 systems
>> >> >   which have broken ACPI video backlight control, the parameter can be
>> >> >   set to 1 in kernel cmdline to skip registering ACPI video's backlight
>> >> >   interface. Due to this change, the acpi_video_verify_backlight_support
>> >> >   is moved from video_detect.c to video.c - patch 3/4;
>> >>
>> >> That's a fairly untenable position for distro kernels to be in.  They
>> >> now have to ask every user that reports an issue with the backlight to
>> >> try setting that option on the command line.  While I appreciate the
>> >> setting breaks things for some people, doesn't the Win8 issue impact
>> >> far more people?  Shouldn't it be defaulted to true?
>> >
>> > Well, we have a rule in the kernel not to introduce regressions for users even
>> > if they are minority.
>> >
>> >> If nothing else, can you add a config option for the default so
>> >> distros can use that to decide which way to default it and then work
>> >> on fixing the remaining users that have troubles?
>> >
>> > The current plan is to create a blacklist of systems where that option should
>> > be set.  We actually already have one, but it is at the _OSI() level, which
>> > is overkill in my view and may affect things beyond backlight.  Along with that
>> > we will debug systems where setting that option (to true) causes problems to
>> > happen, so that we'll be able to drop it going forward (hopefully).
>> >
>> > Of course, distro kernels may always change the default to true if they want.
>>
>> They can, but they'd need to either patch the kernel to do so, or code
>> it in userspace bootloader configs.  Having a config option they can
>> set to change the default makes it reasonable and contained within the
>> kernel.
>
> If we are to use a Kconfig option, why don't we use one instead of rather than
> in addition to a command line option?  Say, we have
> CONFIG_ACPI_VIDEO_WIN8_WORKAROUND and if that is set, the code will work like
> the previous version of the Aaron's patchset (the one without
> video.use_native_backlight)?
>
> Opinions?

If you only have a config option, users can't override the distro
settings.  If you simply have a config option for the default value,
the distros can set it without having to carry a patch (the primary
benefit), but users can still override that without having to rebuild
a kernel.

josh