Re: [REGRESSION BISECTED] backlight control stops workin with 3.14 and later

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 08/22/2014 08:48 AM, Jani Nikula wrote:
> On Fri, 22 Aug 2014, Bertrik Sikken <bertrik@sikken.nl> wrote:
>> On 19-8-2014 3:29, Jani Nikula wrote:
>>> On Tue, 19 Aug 2014, Bertrik Sikken <bertrik@sikken.nl> wrote:
>>>>> On Sun, 17 Aug 2014, Bertrik Sikken <bertrik@sikken.nl> wrote:
>>>>>> On 15-8-2014 3:43, Jani Nikula wrote:
>>>>>>> On Thu, 14 Aug 2014, Bertrik Sikken <bertrik@sikken.nl> wrote:
>>>>>>
>>>>>>>> Attached is dmesg output from booting kernel 3.14-2 (debian unstable)
>>>>>>>> with drm.debug=0xe and the samsung_laptop module enabled, from my
>>>>>>>> Samsung N150plus netbook.
>>>>>>>
>>>>>>> Have you tried 3.15?
>>>>>>
>>>>>> I've built the v3.15 kernel (using the .config file from debian
>>>>>> unstable and doing make oldconfig).
>>>>>>
>>>>>> The backlight is at maximum brightness after boot and I can't control
>>>>>> it using the backlight buttons, nor by writing to
>>>>>> /sys/class/backlight/samsung/brightness
>>>>>> (say half the value or 1/10th of max_brightness)
>>>>>>
>>>>>> Backlight does work when writing
>>>>>> /sys/class/backlight/intel_backlight/brightness
>>>>>
>>>>> How about disabling samsung backlight module with 3.15?
>>>>
>>>> I'm not sure what you mean by that.
>>>>
>>>> As I understand it, there are three ways to control the backlight on this
>>>> netbook: using intel_backlight, samsung_laptop (using a "sabi" interface)
>>>> and acpi_video.
>>>> Backlight control using the samsung_laptop driver no longer seems to work
>>>> after the change. If I disable it (e.g. by blacklisting it), I expect it
>>>> to no longer work at all obviously.
>>>>
>>>> What do you want me to test exactly?
>>>
>>> If the intel_backlight interface works in 3.15, I presume the problem is
>>> that you have a non-functional samsung backlight interface that is
>>> preferred over intel_backlight by your userspace.
>>
>> I tested the intel_backlight interface in linux v3.15 with the samsung
>> backlight module blacklisted. The intel_backlight interface still works.
> 
> I read that as, "I no longer have problems with backlight".

No, just NO. This thread has "REGRESSION" in the subject for a reason,
userspace will prefer a firmware backlight interface over a raw backlight
interface, so the only way to get userspace to use the intel backlight
driver is to blacklist the samsung_laptop and acpi_video (Bertrik can you
confirm that acpi_video also is broken?) so that the intel one is the only
remaining.

Now in some cases that is the right answer, it is actually what we do for
all laptops with win8 "ready" BIOS-es as the firmware interfaces tend to
be broken there, but that is not the case here.

The samsung_laptop backlight interface used to work, and at a minimum we
need to figure out exactly _why_ it stopped working.

We could possibly fix things for Bertrik by a dmi based quirk, but this
problem will likely apply to all samsung netbook devices from the same
era and there are a  _lot_ of different models from those, and the problem
may also apply to other vendor laptops too. Doing a dmi string based quirk is
just papering over the problem, and thus is unacceptable IMHO.

Regards,

Hans






> 
> Jani.
> 
> 
>>
>> Regards,
>> Bertrik
>