Re: [RFC v2 1/2] backlight: pwm_bl: linear interpolation between values of brightness-levels

Daniel Thompson
2017-12-18T13:15:16Z

On Mon, Dec 18, 2017 at 10:47:24AM +0100, Enric Balletbo Serra wrote:
> Hi Daniel,
>
> 2017-12-15 15:40 GMT+01:00 Daniel Thompson <daniel.thompson@linaro.org>:
> > On Thu, Nov 16, 2017 at 03:11:50PM +0100, Enric Balletbo i Serra wrote:
> >>
> >> Setting use-linear-interpolation in the dts will allow you to have linear
> >> interpolation between values of brightness-levels.
> >>
> >> There are now 256 between each of the values of brightness-levels. If
> >> something is requested halfway between 2 values, we'll use linear
> >> interpolation.
> >
> > Why 256?
>
> To be honest there isn't a strong reason, I thought that 256 was a
> good value because is the minimum number of steps possible (8 bits
> pwm). But yeah, guess the discussion is more if this value should be
> calculated, or specified in the the DT more than the value itself.
>
> >>
> >> This way a high resolution pwm duty cycle can be used without having to
> >> list out every possible value in the dts. This system also allows for
> >> gamma corrected values (eg: "brightness-levels = <0 2 4 8 16 32>;").
> >>
> >> Patch based on the Alexandru M Stan work done for ChromeOS kernels.
> >>
> >> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
> >> ---
> >> .../bindings/leds/backlight/pwm-backlight.txt | 2 +
> >> drivers/video/backlight/pwm_bl.c | 55 +++++++++++++++++-----
> >> include/linux/pwm_backlight.h | 2 +
> >> 3 files changed, 47 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> >> index 764db86..7c48f20 100644
> >> --- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> >> +++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> >> @@ -17,6 +17,8 @@ Optional properties:
> >> "pwms" property (see PWM binding[0])
> >> - enable-gpios: contains a single GPIO specifier for the GPIO which enables
> >> and disables the backlight (see GPIO binding[1])
> >> + - use-linear-interpolation: set this propriety to enable linear interpolation
> >> + between each of the values of brightness-levels.
> >
> > Deciding whether or not this deployment of interpolation is a property
> > of the hardware is a finely balanced judgement. On the whole I conclude
> > that since the lookup table is a property of the hardware so too is the
> > deployment of interpolation.
> >
> > Following up on the "why 256?" comment. IMHO either the number of
> > interpolated steps should be calculated from the underlying PWM
> > resolution or it could simply be specified in the DT (e.g. replace
> > use-linear-interpolation with num-interpolated-steps).
> >
>
> Personally I like the idea to have the possibility to specify the
> number of interpolated steps in the DT, I think that will be more
> flexible for the user. If it's ok let me send a first version using
> num-interpolated-steps, and continue the discussion about if makes
> sense to have that in the DT or not.

It's ok from my side.

Daniel.