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

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Date:   Mon, 27 Nov 2017 15:52:37 -0800
From:   Doug Anderson <dianders@...gle.com>
To:     Enric Balletbo i Serra <enric.balletbo@...labora.com>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Richard Purdie <rpurdie@...ys.net>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Pavel Machek <pavel@....cz>, Rob Herring <robh+dt@...nel.org>,
        Brian Norris <briannorris@...gle.com>,
        Guenter Roeck <groeck@...gle.com>,
        Lee Jones <lee.jones@...aro.org>,
        Alexandru Stan <amstan@...gle.com>, linux-leds@...r.kernel.org,
        devicetree@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2 1/2] backlight: pwm_bl: linear interpolation between
 values of brightness-levels

Hi,

On Thu, Nov 16, 2017 at 6:11 AM, Enric Balletbo i Serra
<enric.balletbo@...labora.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@...labora.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?