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

Doug Anderson
2017-11-27T23:52:43Z

Hi,

On Thu, Nov 16, 2017 at 6:11 AM, Enric Balletbo i Serra
<enric.balletbo@collabora.com> 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.
>
> 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.
>
> [0]: Documentation/devicetree/bindings/pwm/pwm.txt
> [1]: Documentation/devicetree/bindings/gpio/gpio.txt
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index 9bd1768..59b1bfb 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -24,6 +24,8 @@
> #include <linux/regulator/consumer.h>
> #include <linux/slab.h>
>
> +#define NSTEPS 256

I'm not sure this is quite the ideal way to specify it. I could sorta
imagine wanting to specify just:

brightness-levels = <0 65535>

...and in such a case you'll only give 256 steps in between. 256
isn't quite granular enough and the human eye can notice each step. I
could fake it by putting this in the device tree:

brightness-levels = <0 4095 8191 ... ... 61439 65535>

...but that's kinda silly. I'd rather just say that when we're using
interpolation we just say that there will be a certain number of steps
(like 32768). Is there really a huge advantage of picking 256 steps
between each specified value instead of just picking a fixed number of
brightness levels?