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

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

Hi,

On 07/22/2014 08:52 AM, Daniel Vetter wrote:
> On Tue, Jul 22, 2014 at 8:42 AM, Hans de Goede <hdegoede@redhat.com> wrote:
>> Hi Jani et al,
>>
>> A friend of mine Bertrik Sikken (in the Cc) his backlight control
>> stopped working for him on his Samsung N150Plus netbook.
>>
>> I took a quick look, and the raw intel_backlight backlight interface
>> works under 3.14, but the firmware samsung_laptop backlight interface,
>> which is what most userspace apps will use by default, stops working
>> in 3.14 .
>>
>> I've asked him to bisect this and the bisect points out this
>> commit as the culprit:
>>
>> b35684b8fa94e04f55fd38bf672b737741d2f9e2 is the first bad commit
>> commit b35684b8fa94e04f55fd38bf672b737741d2f9e2
>> Author: Jani Nikula <jani.nikula@intel.com>
>> Date:   Thu Nov 14 12:13:41 2013 +0200
>>
>>     drm/i915: do full backlight setup at enable time
>>
>>     We should now have all the information we need to do a full
>>     initialization of the backlight registers.
>>
>>     v2: Keep QUIRK_NO_PCH_PWM_ENABLE for now (Imre).
>>
>>     Signed-off-by: Jani Nikula <jani.nikula@intel.com>
>>     Reviewed-by: Imre Deak <imre.deak@intel.com>
>>     Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>>
>> Note that this laptop has an acpi_video backlight interface too,
>> but that has been broken from the start and gets disabled by
>> samsung-laptop based on dmi matching.
> 
> How does the intel backlight fare?

That works fine with 3.14 .

> Please test both 3.14 and 3.15 and

We've tested with 3.14, please let us know if you also want
Bertrik to test with 3.15.

> also test what happens when you blacklist the samsung-laptop driver
> (if that's possible without wreaking the machine).

Then the vendor interface won't get promoted, acpi-video will load,
and things likely will not work.

Bertrik, can you try blacklisting the samsung-laptop module, then
check /sys/class/backlight, the samsung_laptop dir should be gone
replaced by an acpi_video0 (or some such) dir. Please try if that
works. If that does not work, try booting with both the module
blacklisted and "acpi_backlight=vendor" on the kernel commandline,
then you should see only the intel-backlight under /sys/class/backlight
and things should work.

> Also please grab latest intel-gpu-tools and record a register dump
> with intel_reg_dump, again for both broken and working kernels.

Bertrik, can you do this please (without the blacklisting or special
kernel commandline options).

Regards,

Hans