[RFC v2 2/2] backlight: pwm_bl: compute brightness of LED linearly to human eye.

Enric Balletbo i Serra
2017-11-16T14:12:22Z

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 adds support to compute the brightness levels based on a static
table filled with the numbers provided by the CIE 1931 algorithm, for now
it only supports PWM resolutions up to 65535 (16 bits) with 1024 steps.
Lower PWM resolutions are implemented using the same curve but with less
steps, e.g. For a PWM resolution of 256 (8 bits) we have 37 steps.

The calculation of the duty cycle 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>
---
drivers/video/backlight/pwm_bl.c | 160 +++++++++++++++++++++++++++++++++++----
include/linux/pwm_backlight.h | 1 +
2 files changed, 147 insertions(+), 14 deletions(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 59b1bfb..ea96358 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -26,6 +26,112 @@

#define NSTEPS 256

+/*
+ * CIE lightness to PWM conversion table. 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 (output) between 0.0 and 1.0, and L* is the
+ * lightness (input) between 0 and 100.
+ */
+const unsigned int cie1931_table[1024] = {
+ 0, 7, 14, 21, 28, 35, 43, 50, 57, 64, 71, 78, 85, 92, 99, 106, 114, 121,
+ 128, 135, 142, 149, 156, 163, 170, 177, 185, 192, 199, 206, 213, 220,
+ 227, 234, 241, 248, 256, 263, 270, 277, 284, 291, 298, 305, 312, 319,
+ 327, 334, 341, 348, 355, 362, 369, 376, 383, 390, 398, 405, 412, 419,
+ 426, 433, 440, 447, 454, 461, 469, 476, 483, 490, 497, 504, 511, 518,
+ 525, 532, 540, 547, 554, 561, 568, 575, 582, 589, 596, 603, 610, 618,
+ 625, 633, 640, 648, 655, 663, 671, 679, 687, 695, 703, 711, 719, 727,
+ 735, 744, 752, 761, 769, 778, 786, 795, 804, 813, 822, 831, 840, 849,
+ 858, 867, 876, 886, 895, 905, 914, 924, 934, 943, 953, 963, 973, 983,
+ 993, 1004, 1014, 1024, 1034, 1045, 1055, 1066, 1077, 1087, 1098, 1109,
+ 1120, 1131, 1142, 1153, 1165, 1176, 1187, 1199, 1210, 1222, 1234, 1245,
+ 1257, 1269, 1281, 1293, 1305, 1318, 1330, 1342, 1355, 1367, 1380, 1392,
+ 1405, 1418, 1431, 1444, 1457, 1470, 1483, 1497, 1510, 1523, 1537, 1551,
+ 1564, 1578, 1592, 1606, 1620, 1634, 1648, 1662, 1677, 1691, 1706, 1720,
+ 1735, 1750, 1765, 1780, 1795, 1810, 1825, 1840, 1855, 1871, 1886, 1902,
+ 1918, 1933, 1949, 1965, 1981, 1997, 2014, 2030, 2046, 2063, 2079, 2096,
+ 2113, 2130, 2146, 2163, 2181, 2198, 2215, 2232, 2250, 2267, 2285, 2303,
+ 2321, 2338, 2356, 2375, 2393, 2411, 2429, 2448, 2466, 2485, 2504, 2523,
+ 2542, 2561, 2580, 2599, 2618, 2638, 2657, 2677, 2697, 2716, 2736, 2756,
+ 2776, 2796, 2817, 2837, 2858, 2878, 2899, 2920, 2941, 2961, 2983, 3004,
+ 3025, 3046, 3068, 3089, 3111, 3133, 3155, 3177, 3199, 3221, 3243, 3266,
+ 3288, 3311, 3333, 3356, 3379, 3402, 3425, 3448, 3472, 3495, 3519, 3542,
+ 3566, 3590, 3614, 3638, 3662, 3686, 3711, 3735, 3760, 3784, 3809, 3834,
+ 3859, 3884, 3910, 3935, 3960, 3986, 4012, 4037, 4063, 4089, 4115, 4142,
+ 4168, 4194, 4221, 4248, 4274, 4301, 4328, 4356, 4383, 4410, 4438, 4465,
+ 4493, 4521, 4549, 4577, 4605, 4633, 4661, 4690, 4719, 4747, 4776, 4805,
+ 4834, 4863, 4893, 4922, 4952, 4981, 5011, 5041, 5071, 5101, 5131, 5162,
+ 5192, 5223, 5254, 5285, 5316, 5347, 5378, 5409, 5441, 5472, 5504, 5536,
+ 5568, 5600, 5632, 5664, 5697, 5729, 5762, 5795, 5828, 5861, 5894, 5928,
+ 5961, 5995, 6028, 6062, 6096, 6130, 6164, 6199, 6233, 6268, 6302, 6337,
+ 6372, 6407, 6442, 6478, 6513, 6549, 6585, 6621, 6657, 6693, 6729, 6765,
+ 6802, 6839, 6875, 6912, 6949, 6986, 7024, 7061, 7099, 7137, 7174, 7212,
+ 7250, 7289, 7327, 7366, 7404, 7443, 7482, 7521, 7560, 7600, 7639, 7679,
+ 7718, 7758, 7798, 7838, 7879, 7919, 7960, 8000, 8041, 8082, 8123, 8165,
+ 8206, 8248, 8289, 8331, 8373, 8415, 8457, 8500, 8542, 8585, 8628, 8671,
+ 8714, 8757, 8800, 8844, 8887, 8931, 8975, 9019, 9064, 9108, 9152, 9197,
+ 9242, 9287, 9332, 9377, 9423, 9468, 9514, 9560, 9606, 9652, 9698, 9745,
+ 9791, 9838, 9885, 9932, 9979, 10026, 10074, 10121, 10169, 10217, 10265,
+ 10313, 10362, 10410, 10459, 10508, 10557, 10606, 10655, 10704, 10754,
+ 10804, 10854, 10904, 10954, 11004, 11055, 11105, 11156, 11207, 11258,
+ 11310, 11361, 11413, 11464, 11516, 11568, 11620, 11673, 11725, 11778,
+ 11831, 11884, 11937, 11990, 12044, 12097, 12151, 12205, 12259, 12314,
+ 12368, 12423, 12477, 12532, 12587, 12643, 12698, 12754, 12809, 12865,
+ 12921, 12977, 13034, 13090, 13147, 13204, 13261, 13318, 13375, 13433,
+ 13491, 13548, 13606, 13665, 13723, 13781, 13840, 13899, 13958, 14017,
+ 14077, 14136, 14196, 14256, 14316, 14376, 14436, 14497, 14557, 14618,
+ 14679, 14740, 14802, 14863, 14925, 14987, 15049, 15111, 15173, 15236,
+ 15299, 15362, 15425, 15488, 15551, 15615, 15679, 15743, 15807, 15871,
+ 15935, 16000, 16065, 16130, 16195, 16260, 16326, 16392, 16457, 16523,
+ 16590, 16656, 16723, 16789, 16856, 16923, 16991, 17058, 17126, 17194,
+ 17261, 17330, 17398, 17467, 17535, 17604, 17673, 17742, 17812, 17881,
+ 17951, 18021, 18091, 18162, 18232, 18303, 18374, 18445, 18516, 18588,
+ 18659, 18731, 18803, 18875, 18947, 19020, 19093, 19166, 19239, 19312,
+ 19385, 19459, 19533, 19607, 19681, 19755, 19830, 19905, 19980, 20055,
+ 20130, 20206, 20281, 20357, 20433, 20510, 20586, 20663, 20740, 20817,
+ 20894, 20971, 21049, 21127, 21205, 21283, 21361, 21440, 21519, 21598,
+ 21677, 21756, 21836, 21915, 21995, 22075, 22156, 22236, 22317, 22398,
+ 22479, 22560, 22642, 22723, 22805, 22887, 22969, 23052, 23135, 23217,
+ 23300, 23384, 23467, 23551, 23635, 23719, 23803, 23887, 23972, 24057,
+ 24142, 24227, 24313, 24398, 24484, 24570, 24656, 24743, 24829, 24916,
+ 25003, 25091, 25178, 25266, 25354, 25442, 25530, 25618, 25707, 25796,
+ 25885, 25974, 26064, 26153, 26243, 26334, 26424, 26514, 26605, 26696,
+ 26787, 26879, 26970, 27062, 27154, 27246, 27338, 27431, 27524, 27617,
+ 27710, 27804, 27897, 27991, 28085, 28179, 28274, 28369, 28463, 28559,
+ 28654, 28749, 28845, 28941, 29037, 29134, 29230, 29327, 29424, 29521,
+ 29619, 29717, 29815, 29913, 30011, 30110, 30208, 30307, 30406, 30506,
+ 30605, 30705, 30805, 30906, 31006, 31107, 31208, 31309, 31410, 31512,
+ 31614, 31716, 31818, 31920, 32023, 32126, 32229, 32332, 32436, 32540,
+ 32644, 32748, 32852, 32957, 33062, 33167, 33272, 33378, 33484, 33590,
+ 33696, 33802, 33909, 34016, 34123, 34230, 34338, 34446, 34554, 34662,
+ 34770, 34879, 34988, 35097, 35206, 35316, 35426, 35536, 35646, 35757,
+ 35867, 35978, 36090, 36201, 36313, 36425, 36537, 36649, 36762, 36874,
+ 36987, 37101, 37214, 37328, 37442, 37556, 37671, 37785, 37900, 38015,
+ 38131, 38246, 38362, 38478, 38594, 38711, 38828, 38945, 39062, 39179,
+ 39297, 39415, 39533, 39651, 39770, 39889, 40008, 40127, 40247, 40367,
+ 40487, 40607, 40728, 40848, 40969, 41091, 41212, 41334, 41456, 41578,
+ 41700, 41823, 41946, 42069, 42193, 42316, 42440, 42564, 42689, 42813,
+ 42938, 43063, 43189, 43314, 43440, 43566, 43692, 43819, 43946, 44073,
+ 44200, 44328, 44456, 44584, 44712, 44840, 44969, 45098, 45227, 45357,
+ 45487, 45617, 45747, 45877, 46008, 46139, 46270, 46402, 46534, 46666,
+ 46798, 46930, 47063, 47196, 47329, 47463, 47596, 47730, 47865, 47999,
+ 48134, 48269, 48404, 48540, 48675, 48811, 48948, 49084, 49221, 49358,
+ 49495, 49633, 49771, 49909, 50047, 50185, 50324, 50463, 50603, 50742,
+ 50882, 51022, 51162, 51303, 51444, 51585, 51726, 51868, 52010, 52152,
+ 52294, 52437, 52580, 52723, 52867, 53010, 53154, 53299, 53443, 53588,
+ 53733, 53878, 54024, 54169, 54315, 54462, 54608, 54755, 54902, 55050,
+ 55197, 55345, 55493, 55642, 55790, 55939, 56089, 56238, 56388, 56538,
+ 56688, 56839, 56989, 57140, 57292, 57443, 57595, 57747, 57900, 58053,
+ 58205, 58359, 58512, 58666, 58820, 58974, 59129, 59284, 59439, 59594,
+ 59750, 59906, 60062, 60218, 60375, 60532, 60689, 60847, 61005, 61163,
+ 61321, 61480, 61639, 61798, 61957, 62117, 62277, 62437, 62598, 62759,
+ 62920, 63081, 63243, 63405, 63567, 63729, 63892, 64055, 64219, 64382,
+ 64546, 64710, 64875, 65039, 65204, 65369, 65535,
+};
+
struct pwm_bl_data {
struct pwm_device *pwm;
struct device *dev;
@@ -38,6 +144,7 @@ struct pwm_bl_data {
unsigned int scale;
bool legacy;
bool piecewise;
+ bool use_lth_table;
int (*notify)(struct device *,
int brightness);
void (*notify_after)(struct device *,
@@ -104,6 +211,8 @@ static int compute_duty_cycle(struct pwm_bl_data *pb, int brightness)
} else {
duty_cycle = scale(pb, pb->levels[brightness]);
}
+ } else if (pb->use_lth_table) {
+ duty_cycle = cie1931_table[brightness];
} else {
duty_cycle = scale(pb, brightness);
}
@@ -169,9 +278,9 @@ static int pwm_backlight_parse_dt(struct device *dev,
/* determine the number of brightness levels */
prop = of_find_property(node, "brightness-levels", &length);
if (!prop)
- return -EINVAL;
-
- data->levels_count = length / sizeof(u32);
+ data->use_lth_table = true;
+ else
+ data->levels_count = length / sizeof(u32);

/* read brightness levels from DT property */
if (data->levels_count > 0) {
@@ -181,24 +290,28 @@ static int pwm_backlight_parse_dt(struct device *dev,
if (!data->levels)
return -ENOMEM;

- ret = of_property_read_u32_array(node, "brightness-levels",
- data->levels,
- data->levels_count);
- if (ret < 0)
- return ret;
-
- ret = of_property_read_u32(node, "default-brightness-level",
- &value);
- if (ret < 0)
- return ret;
+ if (prop) {
+ ret = of_property_read_u32_array(node,
+ "brightness-levels",
+ data->levels,
+ data->levels_count);
+ if (ret < 0)
+ return ret;
+ }

data->piecewise = of_property_read_bool(node,
"use-linear-interpolation");

- data->dft_brightness = value;
data->levels_count--;
}

+ ret = of_property_read_u32(node, "default-brightness-level",
+ &value);
+ if (ret < 0)
+ return ret;
+
+ data->dft_brightness = value;
+
if (data->piecewise)
data->max_brightness = data->levels_count * NSTEPS;
else
@@ -260,8 +373,10 @@ 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;
int ret;
+ int i;

if (!data) {
ret = pwm_backlight_parse_dt(&pdev->dev, &defdata);
@@ -303,6 +418,7 @@ static int pwm_backlight_probe(struct platform_device *pdev)
pb->dev = &pdev->dev;
pb->enabled = false;
pb->piecewise = data->piecewise;
+ pb->use_lth_table = data->use_lth_table;

pb->enable_gpio = devm_gpiod_get_optional(&pdev->dev, "enable",
GPIOD_ASIS);
@@ -361,6 +477,22 @@ static int pwm_backlight_probe(struct platform_device *pdev)

dev_dbg(&pdev->dev, "got pwm for backlight\n");

+ if (pb->use_lth_table) {
+ /* Get PWM resolution */
+ pwm_get_state(pb->pwm, &state);
+ if (state.period > 65535) {
+ dev_err(&pdev->dev, "PWM resolution not supported\n");
+ goto err_alloc;
+ }
+ /* Find the number of steps based on the PWM resolution */
+ for (i = 0; i < ARRAY_SIZE(cie1931_table); i++)
+ if (cie1931_table[i] >= state.period) {
+ data->max_brightness = i;
+ break;
+ }
+ pb->scale = data->max_brightness;
+ }
+
/*
* FIXME: pwm_apply_args() should be removed when switching to
* the atomic PWM API.
diff --git a/include/linux/pwm_backlight.h b/include/linux/pwm_backlight.h
index 444a91b..4ec3b0d 100644
--- a/include/linux/pwm_backlight.h
+++ b/include/linux/pwm_backlight.h
@@ -15,6 +15,7 @@ struct platform_pwm_backlight_data {
unsigned int pwm_period_ns;
unsigned int *levels;
unsigned int levels_count;
+ bool use_lth_table;
bool piecewise;
/* TODO remove once all users are switched to gpiod_* API */
int enable_gpio;
--
2.9.3