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Message-ID: <CAGF9J3t4_Ru10=x3awb7MeueC3JGhjE4=EfTv7CfU_7tZVFFkA@mail.gmail.com>
Date: Wed, 21 Oct 2020 20:51:15 -0700
From: Alexandru Stan <amstan@...omium.org>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Thierry Reding <thierry.reding@...il.com>,
Uwe Kleine-König
<u.kleine-koenig@...gutronix.de>, Lee Jones <lee.jones@...aro.org>,
Daniel Thompson <daniel.thompson@...aro.org>,
Jingoo Han <jingoohan1@...il.com>,
Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
Heiko Stuebner <heiko@...ech.de>,
Rob Herring <robh+dt@...nel.org>,
Andy Gross <agross@...nel.org>,
Bjorn Andersson <bjorn.andersson@...aro.org>,
Douglas Anderson <dianders@...omium.org>,
Enric Balletbo i Serra <enric.balletbo@...labora.com>,
Matthias Kaehlcke <mka@...omium.org>,
DRI Development <dri-devel@...ts.freedesktop.org>,
Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linux PWM List <linux-pwm@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] backlight: pwm_bl: Fix interpolation
On Wed, 14 Oct 2020 at 23:55, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>
> Hi Alexandru,
>
> On Tue, Oct 13, 2020 at 1:57 PM Alexandru Stan <amstan@...omium.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@...omium.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@...ux-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)
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