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

[Alexandru Stan]

On Wed, 14 Oct 2020 at 23:55, Geert Uytterhoeven <geert at linux-m68k.org> wrote:
>
> Hi Alexandru,
>
> On Tue, Oct 13, 2020 at 1:57 PM Alexandru Stan <amstan at 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 at chromium.org>
>
> Thanks for your patch!

Thanks for your reply!

I'm sorry I haven't replied earlier. Looks like your reply was marked as spam.
Rest be assured my spam filter has been disciplined! :D

>
> > --- 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?

It won't hurt. But it really doesn't make much of a difference either way.

>
> 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

Sure, it might be a little faster to use Bresenham's line algorithm.
Looks like to implement it I would have to deal with some fixed point
math and still have to do divisions occasionally.
I don't think performance is critical here, the values get calculated
only once when the driver loads, and the algorithm's accuracy
improvements might be at most 1 LSB.

Meanwhile the formula I already implemented is almost the same as the
formulas found at
https://en.wikipedia.org/wiki/Linear_interpolation#:~:text=gives
I would like to keep it as is, as straightforward as possible.

>
> >                                 }
> >                         }
> > -                       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 at 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

Alexandru Stan (amstan)