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@xxxxxxxxxx>: > > 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@xxxxxxxxxxxxx> > >> --- > >> .../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. -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
