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Specifying PWM information for devices
======================================
1) PWM user nodes
-----------------
PWM users should specify a list of PWM devices that they want to use
with a property containing a 'pwm-list':
pwm-list ::= <single-pwm> [pwm-list]
single-pwm ::= <pwm-phandle> <pwm-specifier>
pwm-phandle : phandle to PWM controller node
pwm-specifier : array of #pwm-cells specifying the given PWM
(controller specific)
PWM properties should be named "pwms". The exact meaning of each pwms
property must be documented in the device tree binding for each device.
An optional property "pwm-names" may contain a list of strings to label
each of the PWM devices listed in the "pwms" property. If no "pwm-names"
property is given, the name of the user node will be used as fallback.
Drivers for devices that use more than a single PWM device can use the
"pwm-names" property to map the name of the PWM device requested by the
pwm_get() call to an index into the list given by the "pwms" property.
The following example could be used to describe a PWM-based backlight
device:
pwm: pwm {
#pwm-cells = <2>;
};
[...]
bl: backlight {
pwms = <&pwm 0 5000000>;
pwm-names = "backlight";
};
Note that in the example above, specifying the "pwm-names" is redundant
because the name "backlight" would be used as fallback anyway.
pwm-specifier typically encodes the chip-relative PWM number and the PWM
period in nanoseconds.
Optionally, the pwm-specifier can encode a number of flags in a third cell:
- bit 0: PWM signal polarity (0: normal polarity, 1: inverse polarity)
Example with optional PWM specifier for inverse polarity
bl: backlight {
pwms = <&pwm 0 5000000 1>;
pwm-names = "backlight";
};
2) PWM controller nodes
-----------------------
PWM controller nodes must specify the number of cells used for the
specifier using the '#pwm-cells' property.
An example PWM controller might look like this:
pwm: pwm@7000a000 {
compatible = "nvidia,tegra20-pwm";
reg = <0x7000a000 0x100>;
#pwm-cells = <2>;
};
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