On Tue, Oct 11, 2016 at 3:17 AM, Schrempf Frieder
[off-list ref] wrote:

On 10.10.2016 17:20, Rob Herring wrote:

quoted

On Fri, Oct 07, 2016 at 09:08:17AM +0000, Schrempf Frieder wrote:

quoted

This patch adds the documentation for the devicetree bindings to set
the volume levels.

Signed-off-by: Frieder Schrempf <redacted>
---
Changes in v2:
 - split into 3 separate patches
 - make volume properties optional

 .../devicetree/bindings/input/pwm-beeper.txt       | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/Documentation/devicetree/bindings/input/pwm-beeper.txt b/Documentation/devicetree/bindings/input/pwm-beeper.txt
index be332ae..6d8ba4e 100644
--- a/Documentation/devicetree/bindings/input/pwm-beeper.txt
+++ b/Documentation/devicetree/bindings/input/pwm-beeper.txt

@@ -5,3 +5,25 @@ Registers a PWM device as beeper.
 Required properties:
 - compatible: should be "pwm-beeper"
 - pwms: phandle to the physical PWM device
+
+Optional properties:
+- volume-levels: Array of distinct volume levels. These need to be in the
+      range of 0 to 500, while 0 means 0% duty cycle (mute) and 500 means
+      50% duty cycle (max volume).
+      Please note that the actual volume of most beepers is highly
+      non-linear, which means that low volume levels are probably somewhere
+      in the range of 1 to 30 (0.1-3% duty cycle).

What does the index correspond to? The linear volume?

In most cases users probably need linear volume levels (e.g. 0%, 25%,
50%, 75%, 100%) and in this case the index would indeed correspond to
the linear perceived volume.

But also non-linear relations are possible (e.g. 0%, 20%, 100%), if the
user needs for example "mute", "low", "high" as volume levels.

Exclude off/mute and this is still linear. Also, the user exposed
levels could be a subset of the defined h/w levels. That should be
independent of DT.

The linearization (defining the corresponding duty cycle for each index)
depends on the beeper and the perception of the user.

This has to be a consistent interface across h/w to have a userspace
that can work across h/w. For that, you have to define the binding as
linear. Of course, it's all measured by perception and not completely
accurate which is fine.

For the example array definition below, I tried different duty cycles
and found values of 0.8%, 2%, 4%, 50% to be approximately correspondent
to perceived volume levels of 25%, 50%, 75%, 100% in my case.

quoted

quoted

+- default-volume-level: the default volume level (index into the
+      array defined by the "volume-levels" property)
+
+The volume level can be set via sysfs under /sys/class/input/inputX/volume.
+The maximum volume level index can be read from /sys/class/input/inputX/max_volume_level.

Also, drop this. Not relevant to the binding.

Rob