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Diffstat (limited to 'Documentation/driver-api/pwm.rst')
| -rw-r--r-- | Documentation/driver-api/pwm.rst | 61 |
1 files changed, 41 insertions, 20 deletions
diff --git a/Documentation/driver-api/pwm.rst b/Documentation/driver-api/pwm.rst index ab62f1bb0366..b41b1c56477f 100644 --- a/Documentation/driver-api/pwm.rst +++ b/Documentation/driver-api/pwm.rst @@ -35,27 +35,47 @@ consumers to providers, as given in the following example:: 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. +Consumers 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 +the getter, devm_pwm_get() and devm_fwnode_pwm_get(), also exist. After being requested, a PWM has to be configured using:: - int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state); + int pwm_apply_might_sleep(struct pwm_device *pwm, struct pwm_state *state); This API controls both the PWM period/duty_cycle config and the enable/disable state. +PWM devices can be used from atomic context, if the PWM does not sleep. You +can 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's +easily possible, drivers are supposed to emit the inactive state, but some +drivers 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 to +maintain the power output but has more freedom regarding signal form. +If supported by the driver, the signal can be optimized, for example to improve +EMI by phase shifting the individual channels of a chip. + The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers -around pwm_apply_state() and should not be used if the user wants to change +around pwm_apply_might_sleep() and should not be used if the user wants to change several parameter at once. For example, if you see pwm_config() and pwm_{enable,disable}() calls in the same function, this probably means you -should switch to pwm_apply_state(). +should switch to pwm_apply_might_sleep(). -The PWM user API also allows one to query the PWM state with pwm_get_state(). +The PWM user API also allows one to query the PWM state that was passed to the +last invocation of pwm_apply_might_sleep() using pwm_get_state(). Note this is +different to what the driver has actually implemented if the request cannot be +satisfied exactly with the hardware in use. There is currently no way for +consumers to get the actually implemented settings. In addition to the PWM state, the PWM API also exposes PWM arguments, which are the reference PWM config one should use on this PWM. @@ -100,13 +120,13 @@ channel that was exported. The following properties will then be available: duty_cycle The active time of the PWM signal (read/write). - Value is in nanoseconds and must be less than the period. + 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 changing - the polarity. The polarity can only be changed if the PWM is not - enabled. Value is the string "normal" or "inversed". + the polarity. + Value is the string "normal" or "inversed". enable Enable/disable the PWM signal (read/write). @@ -123,11 +143,12 @@ 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. +A new PWM controller/chip can be allocated using pwmchip_alloc(), then +registered using pwmchip_add() and removed again with pwmchip_remove(). To undo +pwmchip_alloc() use pwmchip_put(). 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 @@ -151,8 +172,8 @@ consumers should implement it as described in the "Using PWMs" section. 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 +The PWM core list manipulations are protected by a mutex, so pwm_get() +and pwm_put() 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. |