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

["Bertrik Sikken" <bertrik@sikken.nl>]

> 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".

The problem as I see it, is that the bisected backlight change in the
intel-gfx driver caused the samsung_laptop backlight mechanism to break,
while they apparently were able to play nice with each other before.

You could consider having two drivers trying to access the same hardware
as a more general linux kernel bug. As I understand it now, the intel-gfx
driver talks directly to the hardware and the samsung_laptop driver talks
through a BIOS interface (SABI = "Samsung advanced BIOS interface").

I'm not trying to put the blame on any of the two drivers, just looking
for the best solution to a practical problem.

With kind regards,
Bertrik Sikken