diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c index 105f199a656d..9ee4c1b735b2 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.