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¶

Consumers use thepwm_get() function and pass to it the consumer device or aconsumer name.pwm_put() is used to free the PWM device. Managed variants ofthe getter,devm_pwm_get() anddevm_fwnode_pwm_get(), also exist.

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

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

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

PWM devices can be used from atomic context, if the PWM does not sleep. Youcan check if this the case with:

bool pwm_might_sleep(struct pwm_device *pwm);

If false, the PWM can also be configured from atomic context with:

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

As a consumer, don’t rely on the output’s state for a disabled PWM. If it’seasily possible, drivers are supposed to emit the inactive state, but somedrivers cannot. If you rely on getting the inactive state, use .duty_cycle=0,.enabled=true.

There is also a usage_power setting: If set, the PWM driver is only required tomaintain the power output but has more freedom regarding signal form.If supported by the driver, the signal can be optimized, for example to improveEMI by phase shifting the individual channels of a chip.

Thepwm_config(),pwm_enable() andpwm_disable() functions are just wrappersaroundpwm_apply_might_sleep() 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_might_sleep().

The PWM user API also allows one to query the PWM state that was passed to thelast invocation ofpwm_apply_might_sleep() usingpwm_get_state(). Note this isdifferent to what the driver has actually implemented if the request cannot besatisfied exactly with the hardware in use. There is currently no way forconsumers to get the actually implemented settings.

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).structpwm_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 withpwm_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 or equal to 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.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 allocated usingpwmchip_alloc(), thenregistered usingpwmchip_add() and removed again withpwmchip_remove(). To undopwmchip_alloc() usepwmchip_put().pwmchip_add() takes a filled instructpwm_chip as argument which provides a description of the PWM chip, the numberof PWM devices provided by the chip and the chip-specific implementation of thesupported 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_get()andpwm_put() may not be called from an atomic context.Most functions in the PWM consumer API might sleep and so must not be calledfrom atomic context. The notable exception ispwm_apply_atomic() which has thesame semantics aspwm_apply_might_sleep() but can be called from atomic context.(The price for that is that it doesn’t work for all PWM devices, usepwm_might_sleep() to check if a given PWM supports atomic operation.

Locking in the PWM core ensures that callbacks related to a single chip areserialized.

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.