Re: [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED linearly to human eye.

Daniel Thompson
2018-04-06T15:51:53Z

On Thu, Feb 08, 2018 at 12:30:31PM +0100, Enric Balletbo i Serra wrote:
> When you want to change the brightness using a PWM signal, one thing you
> need to consider is how human perceive the brightness. Human perceive
> the brightness change non-linearly, we have better sensitivity at low
> luminance than high luminance, so to achieve perceived linear dimming,
> the brightness must be matches to the way our eyes behave. The CIE 1931
> lightness formula is what actually describes how we perceive light.
>
> This patch computes a default table with the brightness levels filled
> with the numbers provided by the CIE 1931 algorithm, the number of the
> brightness levels is calculated based on the PWM resolution.
>
> The calculation of the table using the CIE 1931 algorithm is enabled by
> default when you do not define the 'brightness-levels' propriety in your
> device tree.
>
> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>

Acked-by: Daniel Thompson <daniel.thompson@linaro.org>

> ---
> Changes since v2:
> - None
> Changes since v1:
> - Fix an horrible and unjustifiable mistake a missing { fortunately
> catched very quick.
>
> drivers/video/backlight/pwm_bl.c | 149 +++++++++++++++++++++++++++++++++++----
> 1 file changed, 136 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index f0a108ab570a..d047d875f251 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -143,6 +143,107 @@ static const struct backlight_ops pwm_backlight_ops = {
> };
>
> #ifdef CONFIG_OF
> +#define PWM_LUMINANCE_SCALE 10000 /* luminance scale */
> +
> +/* An integer based power function */
> +static u64 int_pow(u64 base, int exp)
> +{
> + u64 result = 1;
> +
> + while (exp) {
> + if (exp & 1)
> + result *= base;
> + exp >>= 1;
> + base *= base;
> + }
> +
> + return result;
> +}
> +
> +/*
> + * CIE lightness to PWM conversion.
> + *
> + * The CIE 1931 lightness formula is what actually describes how we perceive
> + * light:
> + * Y = (L* / 902.3) if L* ≤ 0.08856
> + * Y = ((L* + 16) / 116)^3 if L* > 0.08856
> + *
> + * Where Y is the luminance, the amount of light coming out of the screen, and
> + * is a number between 0.0 and 1.0; and L* is the lightness, how bright a human
> + * perceives the screen to be, and is a number between 0 and 100.
> + *
> + * The following function does the fixed point maths needed to implement the
> + * above formula.
> + */
> +static u64 cie1931(unsigned int lightness, unsigned int scale)
> +{
> + u64 retval;
> +
> + lightness *= 100;
> + if (lightness <= (8 * scale)) {
> + retval = DIV_ROUND_CLOSEST_ULL(lightness * 10, 9023);
> + } else {
> + retval = int_pow((lightness + (16 * scale)) / 116, 3);
> + retval = DIV_ROUND_CLOSEST_ULL(retval, (scale * scale));
> + }
> +
> + return retval;
> +}
> +
> +/*
> + * Create a default correction table for PWM values to create linear brightness
> + * for LED based backlights using the CIE1931 algorithm.
> + */
> +static
> +int pwm_backlight_brightness_default(struct device *dev,
> + struct platform_pwm_backlight_data *data,
> + unsigned int period)
> +{
> + unsigned int counter = 0;
> + unsigned int i, n;
> + u64 retval;
> +
> + /*
> + * Count the number of bits needed to represent the period number. The
> + * number of bits is used to calculate the number of levels used for the
> + * brightness-levels table, the purpose of this calculation is have a
> + * pre-computed table with enough levels to get linear brightness
> + * perception. The period is divided by the number of bits so for a
> + * 8-bit PWM we have 255 / 8 = 32 brightness levels or for a 16-bit PWM
> + * we have 65535 / 16 = 4096 brightness levels.
> + *
> + * Note that this method is based on empirical testing on different
> + * devices with PWM of 8 and 16 bits of resolution.
> + */
> + n = period;
> + while (n) {
> + counter += n % 2;
> + n >>= 1;
> + }
> +
> + data->max_brightness = DIV_ROUND_UP(period, counter);
> + data->levels = devm_kcalloc(dev, data->max_brightness,
> + sizeof(*data->levels), GFP_KERNEL);
> + if (!data->levels)
> + return -ENOMEM;
> +
> + /* Fill the table using the cie1931 algorithm */
> + for (i = 0; i < data->max_brightness; i++) {
> + retval = cie1931((i * PWM_LUMINANCE_SCALE) /
> + data->max_brightness, PWM_LUMINANCE_SCALE) *
> + period;
> + retval = DIV_ROUND_CLOSEST_ULL(retval, PWM_LUMINANCE_SCALE);
> + if (retval > UINT_MAX)
> + return -EINVAL;
> + data->levels[i] = (unsigned int)retval;
> + }
> +
> + data->dft_brightness = data->max_brightness / 2;
> + data->max_brightness--;
> +
> + return 0;
> +}
> +
> static int pwm_backlight_parse_dt(struct device *dev,
> struct platform_pwm_backlight_data *data)
> {
> @@ -161,10 +262,13 @@ static int pwm_backlight_parse_dt(struct device *dev,
>
> memset(data, 0, sizeof(*data));
>
> - /* determine the number of brightness levels */
> + /*
> + * Determine the number of brightness levels, if this property is not
> + * set a default table of brightness levels will be used.
> + */
> prop = of_find_property(node, "brightness-levels", &length);
> if (!prop)
> - return -EINVAL;
> + return 0;
>
> data->max_brightness = length / sizeof(u32);
>
> @@ -294,6 +398,14 @@ static int pwm_backlight_parse_dt(struct device *dev,
> {
> return -ENODEV;
> }
> +
> +static
> +int pwm_backlight_brightness_default(struct device *dev,
> + struct platform_pwm_backlight_data *data,
> + unsigned int period)
> +{
> + return -ENODEV;
> +}
> #endif
>
> static int pwm_backlight_initial_power_state(const struct pwm_bl_data *pb)
> @@ -334,7 +446,9 @@ static int pwm_backlight_probe(struct platform_device *pdev)
> struct backlight_device *bl;
> struct device_node *node = pdev->dev.of_node;
> struct pwm_bl_data *pb;
> + struct pwm_state state;
> struct pwm_args pargs;
> + unsigned int i;
> int ret;
>
> if (!data) {
> @@ -359,17 +473,6 @@ static int pwm_backlight_probe(struct platform_device *pdev)
> goto err_alloc;
> }
>
> - if (data->levels) {
> - unsigned int i;
> -
> - for (i = 0; i <= data->max_brightness; i++)
> - if (data->levels[i] > pb->scale)
> - pb->scale = data->levels[i];
> -
> - pb->levels = data->levels;
> - } else
> - pb->scale = data->max_brightness;
> -
> pb->notify = data->notify;
> pb->notify_after = data->notify_after;
> pb->check_fb = data->check_fb;
> @@ -436,6 +539,26 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>
> dev_dbg(&pdev->dev, "got pwm for backlight\n");
>
> + if (!data->levels) {
> + /* Get the PWM period (in nanoseconds) */
> + pwm_get_state(pb->pwm, &state);
> +
> + ret = pwm_backlight_brightness_default(&pdev->dev, data,
> + state.period);
> + if (ret < 0) {
> + dev_err(&pdev->dev,
> + "failed to setup default brightness table\n");
> + goto err_alloc;
> + }
> + }
> +
> + for (i = 0; i <= data->max_brightness; i++) {
> + if (data->levels[i] > pb->scale)
> + pb->scale = data->levels[i];
> +
> + pb->levels = data->levels;
> + }
> +
> /*
> * FIXME: pwm_apply_args() should be removed when switching to
> * the atomic PWM API.
> --
> 2.15.1
>