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 codeshould instead register a static mapping that can be used to match PWMconsumers 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 usingpwm_request() and free itafter usage withpwm_free().

New users should use thepwm_get() function and pass to it the consumerdevice or a consumer name.pwm_put() is used to free the PWM device. Managedvariants of these functions,devm_pwm_get() anddevm_pwm_put(), also exist.

After being requested, a PWM has to be configured using:

int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state);

This API controls both the PWM period/duty_cycle config and theenable/disable state.

Thepwm_config(),pwm_enable() andpwm_disable() functions are just wrappersaroundpwm_apply_state() and should not be used if the user wants to changeseveral parameter at once. For example, if you seepwm_config() andpwm_{enable,disable}() calls in the same function, this probably means youshould switch topwm_apply_state().

The PWM user API also allows one to query the PWM state withpwm_get_state().

In addition to the PWM state, the PWM API also exposes PWM arguments, whichare the reference PWM config one should use on this PWM.PWM arguments are usually platform-specific and allows the PWM user to onlycare about dutycycle relatively to the full period (like, duty = 50% of theperiod). struct pwm_args contains 2 fields (period and polarity) and shouldbe used to set the initial PWM config (usually done in the probe functionof the PWM user). PWM arguments are retrieved with pwm_get_args().

All consumers should really be reconfiguring the PWM upon resume asappropriate. This is the only way to ensure that everything is resumed inthe proper order.

Using PWMs with the sysfs interface¶

If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfsinterface is provided to use the PWMs from userspace. It is exposed at/sys/class/pwm/. Each probed PWM controller/chip will be exported aspwmchipN, where N is the base of the PWM chip. Inside the directory youwill 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 thepwmchipN directory it is associated with, where X is the number of thechannel 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 inactivetime 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 changingthe polarity. The polarity can only be changed if the PWM is notenabled. 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. Traditionallythere only has been the barebone API meaning that each driver hasto implement the pwm_*() functions itself. This means that it’s impossibleto have multiple PWM drivers in the system. For this reason it’s mandatoryfor new drivers to use the generic PWM framework.

A new PWM controller/chip can be added usingpwmchip_add() and removedagain withpwmchip_remove().pwmchip_add() takes a filled in structpwm_chip as argument which provides a description of the PWM chip, thenumber of PWM devices provided by the chip and the chip-specificimplementation of the supported PWM operations to the framework.

When implementing polarity support in a PWM driver, make sure to respect thesignal conventions in the PWM framework. By definition, normal polaritycharacterizes a signal starts high for the duration of the duty cycle andgoes low for the remainder of the period. Conversely, a signal with inversedpolarity starts low for the duration of the duty cycle and goes high for theremainder of the period.

Drivers are encouraged to implement ->apply() instead of the legacy->enable(), ->disable() and ->config() methods. Doing that should provideatomicity in the PWM config workflow, which is required when the PWM controlsa critical device (like a regulator).

The implementation of ->get_state() (a method used to retrieve initial PWMstate) is also encouraged for the same reason: letting the PWM user knowabout the current PWM state would allow him to avoid glitches.

Drivers should not implement any power management. In other words,consumers should implement it as described in the “Using PWMs” section.

Locking¶

The PWM core list manipulations are protected by a mutex, sopwm_request()andpwm_free() may not be called from an atomic context. Currently thePWM core does not enforce any locking topwm_enable(),pwm_disable() andpwm_config(), so the calling context is currently driver specific. Thisis 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 severaluse cases freq_hz and duty_percent might be better. Instead of calculatingthis in your driver please consider adding appropriate helpers to the framework.