Hi Thierry,

On Tue, Jun 11, 2019 at 12:28:51PM +0200, Thierry Reding wrote:

On Mon, Jun 10, 2019 at 04:37:38PM -0700, Matthias Kaehlcke wrote:

quoted

Add an optional 'max-brightness' property, which is used to specify
the number of brightness levels (max-brightness + 1) when the node
has no 'brightness-levels' table.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
---
 .../devicetree/bindings/leds/backlight/pwm-backlight.txt       | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
index 64fa2fbd98c9..98f4ba626054 100644
--- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
+++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt

@@ -27,6 +27,9 @@ Optional properties:
                             resolution pwm duty cycle can be used without
                             having to list out every possible value in the
                             brightness-level array.
+  - max-brightness: Maximum brightness value. Used to specify the number of
+                    brightness levels (max-brightness + 1) when the node
+                    has no 'brightness-levels' table.

Back at the time when these bindings were defined we specifically didn't
add this because it was deemed impractical. That is, no real hardware is
actually capable of achieving useful results with a simplified
description like this.

Besides, we already have the num-interpolated-steps property which
should allow you to achieve the same thing:

	brightness-levels = <0 255>;
	default-brightness-level = <1>;
	num-interpolated-steps = <255>;

It doesn't achieve the same. With this configuration the device would
have a table with 256 linearly increasing values, the intended use of
the property is to provide the number of brightness levels to be used
by the CIE 1931 algorithm to compute a brightness scale that is
perceived as linear by the human eye. We could possibly treat a
'brightness-levels' table with only two levels as special case and
get the number of levels from it.

In any case from the discussion on "backlight: pwm_bl: Get number of
brightness levels for CIE 1931 from the device tree" it might not be
necessary to specify the number of levels in the DT.

Though given the original discussion that we had around how backlight
hardware behaves, that doesn't seem like a good choice.