[PATCH v5 0/4] Fix Win8 backlight issue

[Seth Forshee]

On Sat, Oct 12, 2013 at 04:44:30AM -0700, Josh Boyer wrote:
> 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.

It sounds like the blacklist and the default value of the parameter
would be inherently tied together, i.e. the blacklist essentially
overrides the default value for specific machines. So when the config
option were flipped from its default the blacklist wouldn't work
anymore, and you'd need a second blacklist for machines which require
video.use_native_backlight=n. I doubt anyone wants to see that happen,
so I think we have to pick one value or the other for the default and
make it configurable only via the command line.

Seth