On Sat, 11 Jun 2016, Lothar Waßmann wrote:

On Fri, 10 Jun 2016 15:54:49 +0100 Lee Jones wrote:

quoted

On Fri, 10 Jun 2016, Lothar Waßmann wrote:

quoted

On Fri, 10 Jun 2016 08:44:49 +0100 Lee Jones wrote:

quoted

On Fri, 10 Jun 2016, Lothar Waßmann wrote:

quoted

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.

The backlight framework does not (and does not need to) know anything
about PWM duty cycles. Its 'brightness' values are consistently 0 =
dark, max = brightest in either case.

What I'm getting at is; by the look of the documentation, the
brightest setting can either be a duty cycle of 0 or 255.  So what
happens with your new semantics when the duty cycle of 0 represents
the brightest setting and you reach 0?  Didn't you just turn the
backlight off?

As mentioned earlier, disabling the PWM has generally the same result as
setting the duty cycle to 0. The current behaviour is broken in this
case, since setting brightness to 0 with a non-zero duty_cycle as the
first element of brightness-levels, the PWM will be disabled rather than
switched to the given duty cycle.
Disabling the PWM should have the same effect as setting the duty cycle
to 0, so it is safe to check for duty_cycle = 0 to decide whether to
disable the PWM.

I agree with this. BUT, that's not what you're doing is it?

Look at the code you're trying to write:

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
        pwm_backlight_power_off(pb);

Let's say duty_cycle = 0.  In some cases this can mean "turn
brightness up to the *maximum*", but with your new logic you just
turned the backlight *off*.

Conversely, let's say duty_cycle = 255.  In some cases this can mean
"turn the brightness to the *lowest* setting" i.e. *off*. Well your
logic just turned the backlight *on*.

If there is something I'm missing, you're going to have to find a
better way to explain it to me.

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