[PATCH v5 0/4] Fix Win8 backlight issue

[Rafael J. Wysocki]

On Saturday, October 12, 2013 02:53:38 AM Igor Gnatenko wrote:
> On Sat, 2013-10-12 at 00:45 +0200, Rafael J. Wysocki 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.
> > 
> > Well, adding a Kconfig option should be simple enough, but I'm not sure
> > I understand the point.  You'll still need to rebuild the kernel after changing
> > that option.
> > 
> 
> I think he meant that we should have Kconfig option alike
> CONFIG_I915_BACKLIGHT_UNREGISTER which have true or false.
> If it true - unregister acpi backlight as default.
> I think so.

That's my understanding too, but I'm not sure about the benefit.  The only one
seems to be that distros wanting to change the default won't have to carry a
patch for that.

Thanks!

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.