[Intel-gfx] [PATCH v5 0/4] Fix Win8 backlight issue

[Rafael J. Wysocki]

On Saturday, October 12, 2013 08:34:32 AM Seth Forshee wrote:
> 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.

Good point.

Thanks!

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