123 lines
4.7 KiB
Plaintext
Executable File
123 lines
4.7 KiB
Plaintext
Executable File
Pulse Width Modulation (PWM) interface
|
|
|
|
This provides an overview about the Linux PWM interface
|
|
|
|
PWMs are commonly used for controlling LEDs, fans or vibrators in
|
|
cell phones. PWMs with a fixed purpose have no need implementing
|
|
the Linux PWM API (although they could). However, PWMs are often
|
|
found as discrete devices on SoCs which have no fixed purpose. It's
|
|
up to the board designer to connect them to LEDs or fans. To provide
|
|
this kind of flexibility the generic PWM API exists.
|
|
|
|
Identifying PWMs
|
|
----------------
|
|
|
|
Users of the legacy PWM API use unique IDs to refer to PWM devices.
|
|
|
|
Instead of referring to a PWM device via its unique ID, board setup code
|
|
should instead register a static mapping that can be used to match PWM
|
|
consumers to providers, as given in the following example:
|
|
|
|
static struct pwm_lookup board_pwm_lookup[] = {
|
|
PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL,
|
|
50000, PWM_POLARITY_NORMAL),
|
|
};
|
|
|
|
static void __init board_init(void)
|
|
{
|
|
...
|
|
pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
|
|
...
|
|
}
|
|
|
|
Using PWMs
|
|
----------
|
|
|
|
Legacy users can request a PWM device using pwm_request() and free it
|
|
after usage with pwm_free().
|
|
|
|
New users should use the pwm_get() function and pass to it the consumer
|
|
device or a consumer name. pwm_put() is used to free the PWM device. Managed
|
|
variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
|
|
|
|
After being requested, a PWM has to be configured using:
|
|
|
|
int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
|
|
|
|
To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
|
|
|
|
Using PWMs with the sysfs interface
|
|
-----------------------------------
|
|
|
|
If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
|
|
interface is provided to use the PWMs from userspace. It is exposed at
|
|
/sys/class/pwm/. Each probed PWM controller/chip will be exported as
|
|
pwmchipN, where N is the base of the PWM chip. Inside the directory you
|
|
will find:
|
|
|
|
npwm - The number of PWM channels this chip supports (read-only).
|
|
|
|
export - Exports a PWM channel for use with sysfs (write-only).
|
|
|
|
unexport - Unexports a PWM channel from sysfs (write-only).
|
|
|
|
The PWM channels are numbered using a per-chip index from 0 to npwm-1.
|
|
|
|
When a PWM channel is exported a pwmX directory will be created in the
|
|
pwmchipN directory it is associated with, where X is the number of the
|
|
channel that was exported. The following properties will then be available:
|
|
|
|
period - The total period of the PWM signal (read/write).
|
|
Value is in nanoseconds and is the sum of the active and inactive
|
|
time of the PWM.
|
|
|
|
duty_cycle - The active time of the PWM signal (read/write).
|
|
Value is in nanoseconds and must be less than the period.
|
|
|
|
polarity - Changes the polarity of the PWM signal (read/write).
|
|
Writes to this property only work if the PWM chip supports changing
|
|
the polarity. The polarity can only be changed if the PWM is not
|
|
enabled. Value is the string "normal" or "inversed".
|
|
|
|
enable - Enable/disable the PWM signal (read/write).
|
|
0 - disabled
|
|
1 - enabled
|
|
|
|
Implementing a PWM driver
|
|
-------------------------
|
|
|
|
Currently there are two ways to implement pwm drivers. Traditionally
|
|
there only has been the barebone API meaning that each driver has
|
|
to implement the pwm_*() functions itself. This means that it's impossible
|
|
to have multiple PWM drivers in the system. For this reason it's mandatory
|
|
for new drivers to use the generic PWM framework.
|
|
|
|
A new PWM controller/chip can be added using pwmchip_add() and removed
|
|
again with pwmchip_remove(). pwmchip_add() takes a filled in struct
|
|
pwm_chip as argument which provides a description of the PWM chip, the
|
|
number of PWM devices provided by the chip and the chip-specific
|
|
implementation of the supported PWM operations to the framework.
|
|
|
|
When implementing polarity support in a PWM driver, make sure to respect the
|
|
signal conventions in the PWM framework. By definition, normal polarity
|
|
characterizes a signal starts high for the duration of the duty cycle and
|
|
goes low for the remainder of the period. Conversely, a signal with inversed
|
|
polarity starts low for the duration of the duty cycle and goes high for the
|
|
remainder of the period.
|
|
|
|
Locking
|
|
-------
|
|
|
|
The PWM core list manipulations are protected by a mutex, so pwm_request()
|
|
and pwm_free() may not be called from an atomic context. Currently the
|
|
PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
|
|
pwm_config(), so the calling context is currently driver specific. This
|
|
is an issue derived from the former barebone API and should be fixed soon.
|
|
|
|
Helpers
|
|
-------
|
|
|
|
Currently a PWM can only be configured with period_ns and duty_ns. For several
|
|
use cases freq_hz and duty_percent might be better. Instead of calculating
|
|
this in your driver please consider adding appropriate helpers to the framework.
|