Re: [PATCH v2 1/3] backlight: pwm_bl: Fix interpolation

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Alexandru,

On Tue, Oct 13, 2020 at 1:57 PM Alexandru Stan <amstan@chromium.org> wrote:
> Whenever num-interpolated-steps was larger than the distance
> between 2 consecutive brightness levels the table would get really
> discontinuous. The slope of the interpolation would stick with
> integers only and if it was 0 the whole line segment would get skipped.
>
> Example settings:
>         brightness-levels = <0 1 2 4 8 16 32 64 128 256>;
>         num-interpolated-steps = <16>;
>
> The distances between 1 2 4 and 8 would be 1, and only starting with 16
> it would start to interpolate properly.
>
> Let's change it so there's always interpolation happening, even if
> there's no enough points available (read: values in the table would
> appear more than once). This should match the expected behavior much
> more closely.
>
> Signed-off-by: Alexandru Stan <amstan@chromium.org>

Thanks for your patch!

> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -327,24 +324,25 @@ static int pwm_backlight_parse_dt(struct device *dev,
>                         table = devm_kzalloc(dev, size, GFP_KERNEL);
>                         if (!table)
>                                 return -ENOMEM;
> -
> -                       /* Fill the interpolated table. */
> -                       levels_count = 0;
> -                       for (i = 0; i < data->max_brightness - 1; i++) {
> -                               value = data->levels[i];
> -                               n = (data->levels[i + 1] - value) / num_steps;
> -                               if (n > 0) {
> -                                       for (j = 0; j < num_steps; j++) {
> -                                               table[levels_count] = value;
> -                                               value += n;
> -                                               levels_count++;
> -                                       }
> -                               } else {
> -                                       table[levels_count] = data->levels[i];
> -                                       levels_count++;
> +                       /*
> +                        * Fill the interpolated table[x] = y
> +                        * by draw lines between each (x1, y1) to (x2, y2).
> +                        */
> +                       dx = num_steps;
> +                       for (i = 0; i < num_input_levels - 1; i++) {
> +                               x1 = i * dx;
> +                               x2 = x1 + dx;
> +                               y1 = data->levels[i];
> +                               y2 = data->levels[i + 1];
> +                               dy = (s64)y2 - y1;
> +
> +                               for (x = x1; x < x2; x++) {
> +                                       table[x] = y1 +
> +                                               div_s64(dy * ((s64)x - x1), dx);

Yummy, 64-by-32 divisions.
Shouldn't this use a rounded division?

Nevertheless, I think it would be worthwhile to implement this using
a (modified) Bresenham algorithm, avoiding multiplications and
divisions, and possibly increasing accuracy as well.

https://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm

>                                 }
>                         }
> -                       table[levels_count] = data->levels[i];
> +                       /* Fill in the last point, since no line starts here. */
> +                       table[x2] = y2;
>
>                         /*
>                          * As we use interpolation lets remove current

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds