Hi Thierry, On Tue, Jun 11, 2019 at 12:28:51PM +0200, Thierry Reding wrote: > On Mon, Jun 10, 2019 at 04:37:38PM -0700, Matthias Kaehlcke wrote: > > Add an optional 'max-brightness' property, which is used to specify > > the number of brightness levels (max-brightness + 1) when the node > > has no 'brightness-levels' table. > > > > Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx> > > --- > > .../devicetree/bindings/leds/backlight/pwm-backlight.txt | 3 +++ > > 1 file changed, 3 insertions(+) > > > > diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt > > index 64fa2fbd98c9..98f4ba626054 100644 > > --- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt > > +++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt > > @@ -27,6 +27,9 @@ Optional properties: > > resolution pwm duty cycle can be used without > > having to list out every possible value in the > > brightness-level array. > > + - max-brightness: Maximum brightness value. Used to specify the number of > > + brightness levels (max-brightness + 1) when the node > > + has no 'brightness-levels' table. > > Back at the time when these bindings were defined we specifically didn't > add this because it was deemed impractical. That is, no real hardware is > actually capable of achieving useful results with a simplified > description like this. > > Besides, we already have the num-interpolated-steps property which > should allow you to achieve the same thing: > > brightness-levels = <0 255>; > default-brightness-level = <1>; > num-interpolated-steps = <255>; It doesn't achieve the same. With this configuration the device would have a table with 256 linearly increasing values, the intended use of the property is to provide the number of brightness levels to be used by the CIE 1931 algorithm to compute a brightness scale that is perceived as linear by the human eye. We could possibly treat a 'brightness-levels' table with only two levels as special case and get the number of levels from it. In any case from the discussion on "backlight: pwm_bl: Get number of brightness levels for CIE 1931 from the device tree" it might not be necessary to specify the number of levels in the DT. > Though given the original discussion that we had around how backlight > hardware behaves, that doesn't seem like a good choice.
