Date:	Wed, 31 Jul 2013 09:59:54 +0800
From:	Aaron Lu <aaron.lwe@...il.com>
To:	Felipe Contreras <felipe.contreras@...il.com>
CC:	"Rafael J. Wysocki" <rjw@...k.pl>, linux-kernel@...r.kernel.org,
	linux-acpi@...r.kernel.org, Len Brown <lenb@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [REGRESSION/PATCH] acpi: blacklist win8 OSI for ASUS Zenbok Prime
 UX31A

On 07/31/2013 04:59 AM, Felipe Contreras wrote:
> On Tue, Jul 30, 2013 at 8:42 AM, Rafael J. Wysocki <rjw@...k.pl> wrote:
>> On Tuesday, July 30, 2013 01:57:55 PM Aaron Lu wrote:
>>> On 07/30/2013 01:51 PM, Aaron Lu wrote:
>>>> On 07/30/2013 11:44 AM, Felipe Contreras wrote:
>>>>> On Mon, Jul 29, 2013 at 10:11 PM, Aaron Lu <aaron.lwe@...il.com> wrote:
>>>>>> On 07/30/2013 03:20 AM, Felipe Contreras wrote:
>>>>>>> Since v3.7 the acpi backlight driver doesn't work at all on this machine
>>>>>>> because presumably the ACPI code contains stub code when Windows 8 OSI is
>>>>>>> reported.
>>>>>>>
>>>>>>> The commit ea45ea7 (in v3.11-rc2) tried to fix this problem by using the intel
>>>>>>> backlight driver, however, on this machine it turns the backlight completely
>>>>>>> off when it reaches level 0%, after which the user might have a lot trouble
>>>>>>> trying to bring it back.
>>>>>>
>>>>>> What do you mean by a lot of trouble? If you press hotkey to increase
>>>>>> backlight brightness level, does it work?
>>>>>
>>>>> I guess so, *if* there is indeed a user-space power manager handling
>>>>> that, *and* the keyboard has such keys, *or* the user has set custom
>>>>> hotkeys.
>>>>
>>>> Right, for a GUI environment this may not be a big problem(user uses Fn
>>>> key to decrease brightness level and then hit the black screen, it's
>>>> natural he will use Fn key to increase brightness level).
>>>>
>>>>>
>>>>>> If so, the screen should not
>>>>>> be black any more, it's not that user has to blindly enter some command
>>>>>> to get out of the black screen.
>>>>>>
>>>>>> And I'm not sure if this is a bug of intel_backlight(setting a low level
>>>>>> makes the screen almost off), I see different models with different
>>>>>> vendors having this behavior.
>>>>>
>>>>> I mean, the screen is completely off, I cannot see absolutely
>>>>> anything. I don't see this behavior with the ACPI backlight driver,
>>>>> nor do I see that in Windows 7.
>>>>>
>>>>>> If this is deemed a bug, then I'm afraid
>>>>>> intel_backlight interface is useless for a lot of systems...perhaps we
>>>>>> can only say, intel_backlight's interpretation of low levels are
>>>>>> different with ACPI video's, and that's probably why its type is named
>>>>>> as raw :-)
>>>>>
>>>>> Well, a bug is defined as unexpected behavior -- as a user, if I'm
>>>>> changing the brightness of the screen, I certainly don't expect the
>>>>> screen to turn off, and I think that's the expectation from most
>>>>> people. It's the first time I see something like that.
>>>>
>>>> I agree this is kind of un-expected. At the same time, this seems to be
>>>> the normal behavior for intel_backlight. I don't know what the correct
>>>> thing to do here if this is something we want to avoid - modify intel
>>>> backlight handling code not to set too low value or change the user
>>>> space tool not to set a too low value if they are interacting with a
>>>> raw type interface. Neither of them sounds cool... Or, users may get
>>>> used to it, I for example, don't find this to be very annoying, but
>>>> maybe I'm already used to it.
>>>
>>> BTW, for the complete screen off problem, I don't see there is anything
>>> wrong with it from code's point of view. It's not that there is an error
>>> in code or this is a broken hardware that caused the screen off when
>>> setting a very low or 0 brightness level, it is simply the expected
>>> behavior of what this interface can provide. It can really set the
>>> brightness level to minimum(zero) or maximum. Don't get me wrong, I
>>> didn't mean this is a good user experience, I don't know that. I just
>>> don't think this is a program bug, and I don't know if this should be
>>> fixed or not - obviously this interface did what it is asked to do,
>>> correctly.
>>
>> Precisely, user space asks for 0 and the kernel delivers.
>>
>> And there are reasons why 0 should be "screen off", like power management
>> (when you have a policy to dim the screen completely after a period of
>> inactivity, for example).
> 
> There is another interface the turn the screen off.
> 
> If 0 turns the screen off with the intel driver, 0 should turn the
> screen off with the ACPI driver, having inconsistent behavior
> depending on which driver is used is a bug.

I'm not sure of this. Remember the days when we switch the hard disk
driver from IDE to SCSI? The block device name changed from hdx to sdx.
Is this a bug?

> 
> If 0 did not turn off the screen in v3.10, 0 should not turn off the
> screen in v3.11, to do so would be a *regression*.

That depends on how you see it. I believe 0 also turns off the screen in
v3.10 if we are talking about the same driver(intel_backlight).

-Aaron

> 
>> So in my opinion, if that's a problem for anyone, it has to be addressed in
>> user space and if there are any vendors who try to address *that* in their ACPI
>> tables, that's one more reason to avoid using ACPI for backlight control.
> 
> If you think it's the user-space responsibility to deal with kernel
> bugs, I think it's only a matter of time before you receive one of
> these [1].
> 
> "If a change results in user programs breaking, it's a bug in the
> kernel. We never EVER blame the user programs. How hard can this be to
> understand?"
> 
> "WE DO NOT BREAK USERSPACE!"
> 
> [1] https://lkml.org/lkml/2012/12/23/75
> 

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