On Wed, 14 Oct 2020 at 23:55, Geert Uytterhoeven geert@linux-m68k.org wrote:
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!
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.
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 currentGr{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
Alexandru Stan (amstan)