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SubjectRe: [PATCH 4/4] backlight: pwm_bl: Set scale type for brightness curves specified in the DT
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On 13/06/2019 20:43, Matthias Kaehlcke wrote:
> Check if a brightness curve specified in the device tree is linear or
> not and set the corresponding property accordingly. This makes the
> scale type available to userspace via the 'scale' sysfs attribute.
>
> To determine if a curve is linear it is compared to a interpolated linear
> curve between min and max brightness. The curve is considered linear if
> no value deviates more than +/-5% of ${brightness_range} from their
> interpolated value.
>
> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> ---
> drivers/video/backlight/pwm_bl.c | 25 +++++++++++++++++++++++++
> 1 file changed, 25 insertions(+)
>
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index f067fe7aa35d..912407b6d67f 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -404,6 +404,26 @@ int pwm_backlight_brightness_default(struct device *dev,
> }
> #endif
>
> +static bool pwm_backlight_is_linear(struct platform_pwm_backlight_data *data)
> +{
> + unsigned int nlevels = data->max_brightness + 1;
> + unsigned int min_val = data->levels[0];
> + unsigned int max_val = data->levels[nlevels - 1];
> + unsigned int slope = (100 * (max_val - min_val)) / nlevels;

Why 100 (rather than a power of 2)?

It would also be good to have a comment here saying what the maximum
quantization error is. Doesn't have to be over complex just mentioning
something like the following (assuming you agree that its true ;-) ):

Multiplying by XXX means that even in pathalogical cases such as
(max_val - min_val) == nlevels then the error at max_val is less than
1%.

With a suitable comment in the fixed point code:
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>


Daniel.


> + unsigned int margin = (max_val - min_val) / 20; /* 5% */
> + int i;
> +
> + for (i = 1; i < nlevels; i++) {
> + unsigned int linear_value = min_val + ((i * slope) / 100);
> + unsigned int delta = abs(linear_value - data->levels[i]);
> +
> + if (delta > margin)
> + return false;
> + }
> +
> + return true;
> +}
> +
> static int pwm_backlight_initial_power_state(const struct pwm_bl_data *pb)
> {
> struct device_node *node = pb->dev->of_node;
> @@ -567,6 +587,11 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>
> pb->levels = data->levels;
> }
> +
> + if (pwm_backlight_is_linear(data))
> + props.scale = BACKLIGHT_SCALE_LINEAR;
> + else
> + props.scale = BACKLIGHT_SCALE_NON_LINEAR;
> } else if (!data->max_brightness) {
> /*
> * If no brightness levels are provided and max_brightness is
>

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