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Date: Fri, 4 Sep 2020 12:27:32 +0100
From: Daniel Thompson <daniel.thompson@...aro.org>
To: Alexandru Stan <amstan@...omium.org>
Cc: Thierry Reding <thierry.reding@...il.com>,
Uwe Kleine-König
<u.kleine-koenig@...gutronix.de>, Lee Jones <lee.jones@...aro.org>,
Jingoo Han <jingoohan1@...il.com>,
Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
Heiko Stuebner <heiko@...ech.de>,
Rob Herring <robh+dt@...nel.org>,
Matthias Kaehlcke <mka@...omium.org>,
Douglas Anderson <dianders@...omium.org>,
Enric Balletbo i Serra <enric.balletbo@...labora.com>,
dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org
Subject: Re: [PATCH 1/3] backlight: pwm_bl: Fix interpolation
On Mon, Jul 20, 2020 at 09:25:20PM -0700, Alexandru Stan 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.
>
> Reviewed-by: Douglas Anderson <dianders@...omium.org>
> Reviewed-by: Matthias Kaehlcke <mka@...omium.org>
> Signed-off-by: Alexandru Stan <amstan@...omium.org>
Apologies for the delay. Patch 2/3 meant I had some thinking to do...
and then the holiday's took their toll.
Overall this looks good, just some quibbles about broken 64-bit maths.
> ---
>
> drivers/video/backlight/pwm_bl.c | 70 ++++++++++++++------------------
> 1 file changed, 31 insertions(+), 39 deletions(-)
>
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index 82b8d7594701..5193a72305a2 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -235,8 +235,7 @@ static int pwm_backlight_parse_dt(struct device *dev,
> struct platform_pwm_backlight_data *data)
> {
> struct device_node *node = dev->of_node;
> - unsigned int num_levels = 0;
> - unsigned int levels_count;
> + unsigned int num_levels;
> unsigned int num_steps = 0;
> struct property *prop;
> unsigned int *table;
> @@ -265,12 +264,11 @@ static int pwm_backlight_parse_dt(struct device *dev,
> if (!prop)
> return 0;
>
> - data->max_brightness = length / sizeof(u32);
> + num_levels = length / sizeof(u32);
>
> /* read brightness levels from DT property */
> - if (data->max_brightness > 0) {
> - size_t size = sizeof(*data->levels) * data->max_brightness;
> - unsigned int i, j, n = 0;
> + if (num_levels > 0) {
> + size_t size = sizeof(*data->levels) * num_levels;
>
> data->levels = devm_kzalloc(dev, size, GFP_KERNEL);
> if (!data->levels)
> @@ -278,7 +276,7 @@ static int pwm_backlight_parse_dt(struct device *dev,
>
> ret = of_property_read_u32_array(node, "brightness-levels",
> data->levels,
> - data->max_brightness);
> + num_levels);
> if (ret < 0)
> return ret;
>
> @@ -303,7 +301,13 @@ static int pwm_backlight_parse_dt(struct device *dev,
> * between two points.
> */
> if (num_steps) {
> - if (data->max_brightness < 2) {
> + unsigned int num_input_levels = num_levels;
> + unsigned int i;
> + u32 x1, x2, x;
> + u32 y1, y2;
> + s64 dx, dy;
dx should be 32-bit. It will be truncated to 32-bit when passed to
div_s64() so this type is actively misleading about how the maths
works.
> +
> + if (num_input_levels < 2) {
> dev_err(dev, "can't interpolate\n");
> return -EINVAL;
> }
> @@ -313,14 +317,7 @@ static int pwm_backlight_parse_dt(struct device *dev,
> * taking in consideration the number of interpolated
> * steps between two levels.
> */
> - for (i = 0; i < data->max_brightness - 1; i++) {
> - if ((data->levels[i + 1] - data->levels[i]) /
> - num_steps)
> - num_levels += num_steps;
> - else
> - num_levels++;
> - }
> - num_levels++;
> + num_levels = (num_input_levels - 1) * num_steps + 1;
> dev_dbg(dev, "new number of brightness levels: %d\n",
> num_levels);
>
> @@ -332,24 +329,25 @@ 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 = y2 - y1;
This is an u32 expression being assigned to a s64. I could be rusty on
my fixed point maths but won't this promote too late for the 64-bitness
of dy to be useful?
Daniel.
> +
> + for (x = x1; x < x2; x++) {
> + table[x] = y1 +
> + div_s64(dy * (x - x1), dx);
> }
> }
> - 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
> @@ -358,15 +356,9 @@ static int pwm_backlight_parse_dt(struct device *dev,
> */
> devm_kfree(dev, data->levels);
> data->levels = table;
> -
> - /*
> - * Reassign max_brightness value to the new total number
> - * of brightness levels.
> - */
> - data->max_brightness = num_levels;
> }
>
> - data->max_brightness--;
> + data->max_brightness = num_levels - 1;
> }
>
> return 0;
> --
> 2.27.0
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