Re: [PATCH] drm/i915: A better maximum brightness for users.

[Jani Nikula <jani.nikula@linux.intel.com>]

On Mon, 09 Nov 2015, "Shih-Yuan Lee (FourDollars)" <sylee@canonical.com> wrote:
> On Mon, Nov 9, 2015 at 6:51 PM, Jani Nikula <jani.nikula@linux.intel.com>
> wrote:
>
>> On Mon, 09 Nov 2015, "Shih-Yuan Lee (FourDollars)" <sylee@canonical.com>
>> wrote:
>> > On Mon, Nov 9, 2015 at 6:17 PM, Jani Nikula <jani.nikula@linux.intel.com
>> >
>> > wrote:
>> >
>> >> On Mon, 09 Nov 2015, "Shih-Yuan Lee (FourDollars)" <sylee@canonical.com
>> >
>> >> wrote:
>> >> > The PWM brightness level of Dell XPS 13 (2015) is from 10 to 937
>> however
>> >> > the sysfs brightness level always starts from 0 so it is better to use
>> >> > 927 as the sysfs maximum brightness level and it becomes easier to map
>> >> > from the PWM brightness level to the sysfs brightness level.
>> >>
>> >> We've been thinking we should provide a fixed range to userspace
>> >> instead. Say, 0-100.
>> >>
>> >> BR,
>> >> Jani.
>> >>
>> >>
>> >>
>> > That might not be a good idea for the backward compatibility.
>>
>> Please explain how you think your change is good and a fixed range 0-100
>> is bad in terms of backward compatibility. Both just arbitrarily change
>> the max.
>>
> The original sysfs brightness range is from 0 to 937. Let's define it as A
> = {0, 1, 2, ..., 937}.
> The PWM brightness range is from 10 to 937. Let's define it as B = {10, 11,
> 12, ..., 937}.
> |A| = 938, |B| = 928
> |A| != |B|
> It implies A and B is not an 1-1 mapping.
>
> My patch is not a arbitrarily change.
> It makes A' = {0, 1, 2, ..., 927}. |A'| = 928
> You can see |A'| = |B| so it implies A' and B is an 1-1 mapping.
> It means any value in A' can be mapped to a different value in B and visa
> versa.
>
> C = {0, 100} has the same mapping problem.

Please tell me why you think this is a problem to begin with. What (user
perceptible) problem are you trying to solve?

I understand we could simplify (or remove) the scaling code with your
change, but I'm reluctant to go that way when, as I said, we've been
talking about fixing the range reported to userspace.

Part of the reason for going to 0-100 range would be that there are
barely that many user distinguishable steps in the backlight level. It
is silly to have brightness range of, say, 0-937, because you can't
distinguish them from each other. (Perhaps counter-intuitively, the
higher the PWM modulation frequency, the fewer user distinguishable
levels you can actually get.)

>> Besides, we've changed the max for some platforms before, and the ABI
>> for backlight sysfs is you can stick a value between 0 and
>> max_brightness to the brightness attribute. Any userspace that relies on
>> anything else is broken.
>>
>> > However I saw some message as the following.
>> > [    3.402233] [drm:parse_lfp_backlight] VBT backlight PWM modulation
>> > frequency 200 Hz, active high, min brightness 10, level 255
>> >
>> >
>> > Does it mean the brightness range is also defined in the BIOS?
>>
>> The minimum and the PWM modulation frequency are defined in BIOS. The
>> modulation frequency is an attribute for the hardware; I think that was
>> originally exposed as the max was just for implementation convenience.
>>
> I don't mean the minimum or the PWM modulation frequency.
> I mean "level 255".
> Does it mean the brightness range or something else?

It probably means the suggested initial level of the backlight in some
units, but AFAICT we don't use that for anything, and frankly I am not
sure why we log it.

BR,
Jani.


>
> Regards,
> $4
>
>>
>> BR,
>> Jani.
>>
>>
>> >
>> > Regards,
>> > $4
>> >
>> >>
>> >> >
>> >> > Signed-off-by: Shih-Yuan Lee (FourDollars) <sylee@canonical.com>
>> >> > ---
>> >> >  drivers/gpu/drm/i915/intel_panel.c | 2 +-
>> >> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >> >
>> >> > diff --git a/drivers/gpu/drm/i915/intel_panel.c
>> >> b/drivers/gpu/drm/i915/intel_panel.c
>> >> > index a24df35..697fd4d 100644
>> >> > --- a/drivers/gpu/drm/i915/intel_panel.c
>> >> > +++ b/drivers/gpu/drm/i915/intel_panel.c
>> >> > @@ -1211,7 +1211,7 @@ static int
>> intel_backlight_device_register(struct
>> >> intel_connector *connector)
>> >> >        * Note: Everything should work even if the backlight device max
>> >> >        * presented to the userspace is arbitrarily chosen.
>> >> >        */
>> >> > -     props.max_brightness = panel->backlight.max;
>> >> > +     props.max_brightness = panel->backlight.max -
>> panel->backlight.min;
>> >> >       props.brightness = scale_hw_to_user(connector,
>> >> >                                           panel->backlight.level,
>> >> >                                           props.max_brightness);
>> >>
>> >> --
>> >> Jani Nikula, Intel Open Source Technology Center
>> >>
>>
>> --
>> Jani Nikula, Intel Open Source Technology Center
>>

-- 
Jani Nikula, Intel Open Source Technology Center
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