[PATCHv2] backlight: pwm_bl: disable PWM when 'duty_cycle' is zero

Lothar Waßmann
2016-06-07T10:13:49Z

'brightness' is usually an index into a table of duty_cycle values,
where the value at index 0 may well be non-zero
(tegra30-apalis-eval.dts and tegra30-colibri-eval-v3.dts are real-life
examples).
Thus brightness = 0 does not necessarily mean that the PWM output
will be inactive.
Check for 'duty_cycle = 0' rather than 'brightness = 0' to decide
whether to disable the PWM.

Signed-off-by: Lothar WaÃŸmann <LW@KARO-electronics.de>
---
Changes wrt. v1:
- update binding docs to reflect the change

.../devicetree/bindings/leds/backlight/pwm-backlight.txt | 9 ++++++---
drivers/video/backlight/pwm_bl.c | 4 ++--
2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
index 764db86..95fa8a9 100644
--- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
+++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
@@ -4,10 +4,13 @@ Required properties:
- compatible: "pwm-backlight"
- pwms: OF device-tree PWM specification (see PWM binding[0])
- brightness-levels: Array of distinct brightness levels. Typically these
- are in the range from 0 to 255, but any range starting at 0 will do.
+ are in the range from 0 to 255, but any range will do.
The actual brightness level (PWM duty cycle) will be interpolated
- from these values. 0 means a 0% duty cycle (darkest/off), while the
- last value in the array represents a 100% duty cycle (brightest).
+ from these values. 0 means a 0% duty cycle, while the highest value in
+ the array represents a 100% duty cycle.
+ The range may be in reverse order (starting with the maximum duty cycle
+ value) to create a PWM signal with the 100% duty cycle representing
+ minimum and 0% duty cycle maximum brigthness.
- default-brightness-level: the default brightness level (index into the
array defined by the "brightness-levels" property)
- power-supply: regulator for supply voltage
diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index b2b366b..80b2b52 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -103,8 +103,8 @@ static int pwm_backlight_update_status(struct backlight_device *bl)
if (pb->notify)
brightness = pb->notify(pb->dev, brightness);

- if (brightness > 0) {
- duty_cycle = compute_duty_cycle(pb, brightness);
+ duty_cycle = compute_duty_cycle(pb, brightness);
+ if (duty_cycle > 0) {
pwm_config(pb->pwm, duty_cycle, pb->period);
pwm_backlight_power_on(pb, brightness);
} else
--
2.1.4