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Date:	Mon, 14 Jul 2014 15:27:50 +0200
From:	Bjørn Mork <>
To:	Hans de Goede <>
Cc:	Linus Torvalds <>,,,
	"Rafael J. Wysocki" <>
Subject: Re: [BISECTED 3.16-rc REGREGRESSION] backlight control stopped working

Hans de Goede <> writes:

> Hi,
> On 07/14/2014 02:59 PM, Bjørn Mork wrote:
>> Yes, I actually bisected this just to get
>> 886129a8eebebec260165741fe31421482371006 is the first bad commit
>> commit 886129a8eebebec260165741fe31421482371006
>> Author: Hans de Goede <>
>> Date:   Tue May 6 14:46:23 2014 +0200
>>     ACPI / video: change acpi-video brightness_switch_enabled default to 0
>>     acpi-video is unique in that it not only generates brightness up/down
>>     keypresses, but also (sometimes) actively changes the brightness itself.
>>     This presents an inconsistent kernel interface to userspace, basically there
>>     are 2 different scenarios, depending on the laptop model:
>>      1) On some laptops a brightness up/down keypress means: show a brightness osd
>>      with the current brightness, iow it is a brightness has changed notification.
>>      2) Where as on (a lot of) other laptops it means a brightness up/down key was
>>      pressed, deal with it.
>>     Most of the desktop environments interpret any press as in scenario 2, and
>>     change the brightness up / down as a response to the key events, causing it
>>     to be changed twice, once by acpi-video and once by the DE.
>>     With the new default for video.use_native_backlight we will be moving even
>>     more laptops over to behaving as in scenario 2. Making the remaining laptops
>>     even more of a weird exception. Also note that it is hard to detect scenario
>>     1 properly in userspace, and AFAIK none of the DE-s deals with it.
>>     Therefor this commit changes the default of brightness_switch_enabled to 0
>>     making its behavior consistent with all the other backlight drivers.
>>     Signed-off-by: Hans de Goede <>
>>     Reviewed-by: Aaron Lu <>
>>     Signed-off-by: Rafael J. Wysocki <>
>> :040000 040000 5bbac8c4f3e9fd5421daf84289004c32c3217f2a 82c99a358bda6360f845b6063182d3e707ff90f0 M      Documentation
>> :040000 040000 81ed56a41130bbbea980620114ff11e3bb23ee63 a9870ba1d046bde69796060304c78dfbb1d00a1f M      drivers
>> The fact that this seems to be an *intentional* breakage does not help a
>> lot.  Yes, I understand that you believe the choice of default was
>> incorrect for some reason.  You might even be right about that.  But
>> that is still not a valid reason to break existing configurations for
>> end users!  Please do not do that.
> This *not* a regression, it is an intended behavior change

On my laptop it changed the behaviour from working to non-working, which
is a regression whether it was intended or not.

> which can be
> flipped back to its old behavior with a single cmdline argument (add
> video.brightness_switch_enabled=1 to the kernel commandline).

Yes, sure.

But we do not require users to add command line settings to keep
existing behaviour.

For the record, AFAICS, you could achive *your* wanted effect by adding
video.brightness_switch_enabled=0 to the kernel commandline.

> Yes this may break existing configurations for some users, most likely
> users running some custom setup, who thus should be able to fix things
> by adding one more customization to there setup.

You can drop the "may".  I have already told you that it broke my
configuration, haven't I?

> OTOH this will also unbreak a lot of existing setups, likely an amount
> of setups which is at least one order of magnitude bigger then the
> amount it will break.

If so, then would adding video.brightness_switch_enabled=0 to the kernel
commandline on those systems.  Which would have the nice effect that it
wouldn't break anything.

But whavever.  Since when was it OK to intentionally break existing
setups for *any* reason?

> Most users will be running a desktop environment which will react
> to the keypresses (which are always generated in cases where
> brightness_switch_enabled does anything) by changing the brightness
> a second time. This happens at least in gnome, kde, xfce, unity and
> probably in a few smaller desktop environments as configured ootb too.

Now I believe we are moving out on the prairie here.  I am concerned
about the *kernel*, not desktop environments.

> If you've a backlight control which only has 8 steps taking 2 steps
> at a time is a bug issue, see e.g. :
> TL;DR: This change really is for the better and is here to stay.
>> Note that NO USER cares about "some laptops" or "other laptops".  They
>> care about their own systems, which either
>>  a) depend on the old default and therefore breaks with your change, or
>>  b) have a user modified setting and therefore are unaffected by your
>>     change
> This is not how it works, sometimes we *must* look at the bigger picture,
> e.g. when the Linux kernel first started advertising to acpi that it
> was "Windows 2012" (aka win8), this causes some breakage, still we went
> ahead with the change, because in the end we must advertise "Windows 2012"
> support to be able to get support for certain features from the firmware.

Please explain the bigger picture then.  From what I can see, you have
not made a single attempt on explaining why the broken systems cannot be
fixed be fixed by adding video.brightness_switch_enabled=0 to the kernel

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