[PATCH AUTOSEL 4.19 356/671] backlight: lm3630a: Return 0 on success in update_status functions

[PATCH AUTOSEL 4.19 356/671] backlight: lm3630a: Return 0 on success in update_status functions

[Sasha Levin]

From: Brian Masney <masneyb@onstation.org>

[ Upstream commit d3f48ec0954c6aac736ab21c34a35d7554409112 ]

lm3630a_bank_a_update_status() and lm3630a_bank_b_update_status()
both return the brightness value if the brightness was successfully
updated. Writing to these attributes via sysfs would cause a 'Bad
address' error to be returned. These functions should return 0 on
success, so let's change it to correct that error.

Fixes: 28e64a68a2ef ("backlight: lm3630: apply chip revision")
Signed-off-by: Brian Masney <masneyb@onstation.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/video/backlight/lm3630a_bl.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/video/backlight/lm3630a_bl.c b/drivers/video/backlight/lm3630a_bl.c
index 2030a6b77a09..ef2553f452ca 100644
--- a/drivers/video/backlight/lm3630a_bl.c
+++ b/drivers/video/backlight/lm3630a_bl.c
@@ -201,7 +201,7 @@ static int lm3630a_bank_a_update_status(struct backlight_device *bl)
 				      LM3630A_LEDA_ENABLE, LM3630A_LEDA_ENABLE);
 	if (ret < 0)
 		goto out_i2c_err;
-	return bl->props.brightness;
+	return 0;
 
 out_i2c_err:
 	dev_err(pchip->dev, "i2c failed to access\n");
@@ -278,7 +278,7 @@ static int lm3630a_bank_b_update_status(struct backlight_device *bl)
 				      LM3630A_LEDB_ENABLE, LM3630A_LEDB_ENABLE);
 	if (ret < 0)
 		goto out_i2c_err;
-	return bl->props.brightness;
+	return 0;
 
 out_i2c_err:
 	dev_err(pchip->dev, "i2c failed to access REG_CTRL\n");
-- 
2.20.1

[PATCH AUTOSEL 4.19 438/671] backlight: pwm_bl: Fix heuristic to determine number of brightness leve

[Sasha Levin]

From: Matthias Kaehlcke <mka@chromium.org>

[ Upstream commit 73fbfc499448455f1e1c77717040e09e25f1d976 ]

With commit 88ba95bedb79 ("backlight: pwm_bl: Compute brightness of
LED linearly to human eye") the number of set bits (aka hweight())
in the PWM period is used in the heuristic to determine the number
of brightness levels, when the brightness table isn't specified in
the DT. The number of set bits doesn't provide a reliable clue about
the length of the period, instead change the heuristic to:

 nlevels = period / fls(period)

Also limit the maximum number of brightness levels to 4096 to avoid
excessively large tables.

With this the number of levels increases monotonically with the PWM
period, until the maximum of 4096 levels is reached:

period (ns)    # levels

100    	       16
500	       62
1000	       111
5000	       416
10000	       769
50000	       3333
100000	       4096

Fixes: 88ba95bedb79 ("backlight: pwm_bl: Compute brightness of LED linearly to human eye")
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/video/backlight/pwm_bl.c | 24 ++++++------------------
 1 file changed, 6 insertions(+), 18 deletions(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 7ddc0930e98c..3a3098d4873b 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -199,29 +199,17 @@ 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;
+	unsigned int i;
 	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.
+	 * Once we have 4096 levels there's little point going much higher...
+	 * neither interactive sliders nor animation benefits from having
+	 * more values in the table.
 	 */
-	n = period;
-	while (n) {
-		counter += n % 2;
-		n >>= 1;
-	}
+	data->max_brightness +		min((int)DIV_ROUND_UP(period, fls(period)), 4096);
 
-	data->max_brightness = DIV_ROUND_UP(period, counter);
 	data->levels = devm_kcalloc(dev, data->max_brightness,
 				    sizeof(*data->levels), GFP_KERNEL);
 	if (!data->levels)
-- 
2.20.1