On Fri, 10 Jun 2016, Lothar Waßmann wrote:

Hi,

On Thu, 9 Jun 2016 14:51:25 +0100 Lee Jones wrote:

quoted

On Tue, 07 Jun 2016, Lothar Waßmann wrote:

quoted

'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 Waßmann <redacted>
---
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) {

How does this work in the aforementioned:

  "The range may be in reverse order"

... case?  Surely when duty_cycle is when the screen should be at it's
brightest?  Wouldn't it confuse the user if they turn their brightness
*up* and the screen goes *off*?

Assuming that the PWM output is inactive (LOW) when the duty_cycle is
set to zero, there will be no difference between operating the PWM at
duty_cycle 0 or disabling it.

Currently, the screen will go bright when it should be off in this
case.

It sounds like we need something that lets the framework know if
duty_cycle = MAX is the brightest or if duty_cycle = 0 is.  Either way
someone is going to get screwed by this logic.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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