linux/Documentation/pwm.txt
<<
>>
Prefs
   1Pulse Width Modulation (PWM) interface
   2
   3This provides an overview about the Linux PWM interface
   4
   5PWMs are commonly used for controlling LEDs, fans or vibrators in
   6cell phones. PWMs with a fixed purpose have no need implementing
   7the Linux PWM API (although they could). However, PWMs are often
   8found as discrete devices on SoCs which have no fixed purpose. It's
   9up to the board designer to connect them to LEDs or fans. To provide
  10this kind of flexibility the generic PWM API exists.
  11
  12Identifying PWMs
  13----------------
  14
  15Users of the legacy PWM API use unique IDs to refer to PWM devices.
  16
  17Instead of referring to a PWM device via its unique ID, board setup code
  18should instead register a static mapping that can be used to match PWM
  19consumers to providers, as given in the following example:
  20
  21        static struct pwm_lookup board_pwm_lookup[] = {
  22                PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL,
  23                           50000, PWM_POLARITY_NORMAL),
  24        };
  25
  26        static void __init board_init(void)
  27        {
  28                ...
  29                pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
  30                ...
  31        }
  32
  33Using PWMs
  34----------
  35
  36Legacy users can request a PWM device using pwm_request() and free it
  37after usage with pwm_free().
  38
  39New users should use the pwm_get() function and pass to it the consumer
  40device or a consumer name. pwm_put() is used to free the PWM device. Managed
  41variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
  42
  43After being requested, a PWM has to be configured using:
  44
  45int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state);
  46
  47This API controls both the PWM period/duty_cycle config and the
  48enable/disable state.
  49
  50The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
  51around pwm_apply_state() and should not be used if the user wants to change
  52several parameter at once. For example, if you see pwm_config() and
  53pwm_{enable,disable}() calls in the same function, this probably means you
  54should switch to pwm_apply_state().
  55
  56The PWM user API also allows one to query the PWM state with pwm_get_state().
  57
  58In addition to the PWM state, the PWM API also exposes PWM arguments, which
  59are the reference PWM config one should use on this PWM.
  60PWM arguments are usually platform-specific and allows the PWM user to only
  61care about dutycycle relatively to the full period (like, duty = 50% of the
  62period). struct pwm_args contains 2 fields (period and polarity) and should
  63be used to set the initial PWM config (usually done in the probe function
  64of the PWM user). PWM arguments are retrieved with pwm_get_args().
  65
  66Using PWMs with the sysfs interface
  67-----------------------------------
  68
  69If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
  70interface is provided to use the PWMs from userspace. It is exposed at
  71/sys/class/pwm/. Each probed PWM controller/chip will be exported as
  72pwmchipN, where N is the base of the PWM chip. Inside the directory you
  73will find:
  74
  75npwm - The number of PWM channels this chip supports (read-only).
  76
  77export - Exports a PWM channel for use with sysfs (write-only).
  78
  79unexport - Unexports a PWM channel from sysfs (write-only).
  80
  81The PWM channels are numbered using a per-chip index from 0 to npwm-1.
  82
  83When a PWM channel is exported a pwmX directory will be created in the
  84pwmchipN directory it is associated with, where X is the number of the
  85channel that was exported. The following properties will then be available:
  86
  87period - The total period of the PWM signal (read/write).
  88        Value is in nanoseconds and is the sum of the active and inactive
  89        time of the PWM.
  90
  91duty_cycle - The active time of the PWM signal (read/write).
  92        Value is in nanoseconds and must be less than the period.
  93
  94polarity - Changes the polarity of the PWM signal (read/write).
  95        Writes to this property only work if the PWM chip supports changing
  96        the polarity. The polarity can only be changed if the PWM is not
  97        enabled. Value is the string "normal" or "inversed".
  98
  99enable - Enable/disable the PWM signal (read/write).
 100        0 - disabled
 101        1 - enabled
 102
 103Implementing a PWM driver
 104-------------------------
 105
 106Currently there are two ways to implement pwm drivers. Traditionally
 107there only has been the barebone API meaning that each driver has
 108to implement the pwm_*() functions itself. This means that it's impossible
 109to have multiple PWM drivers in the system. For this reason it's mandatory
 110for new drivers to use the generic PWM framework.
 111
 112A new PWM controller/chip can be added using pwmchip_add() and removed
 113again with pwmchip_remove(). pwmchip_add() takes a filled in struct
 114pwm_chip as argument which provides a description of the PWM chip, the
 115number of PWM devices provided by the chip and the chip-specific
 116implementation of the supported PWM operations to the framework.
 117
 118When implementing polarity support in a PWM driver, make sure to respect the
 119signal conventions in the PWM framework. By definition, normal polarity
 120characterizes a signal starts high for the duration of the duty cycle and
 121goes low for the remainder of the period. Conversely, a signal with inversed
 122polarity starts low for the duration of the duty cycle and goes high for the
 123remainder of the period.
 124
 125Drivers are encouraged to implement ->apply() instead of the legacy
 126->enable(), ->disable() and ->config() methods. Doing that should provide
 127atomicity in the PWM config workflow, which is required when the PWM controls
 128a critical device (like a regulator).
 129
 130The implementation of ->get_state() (a method used to retrieve initial PWM
 131state) is also encouraged for the same reason: letting the PWM user know
 132about the current PWM state would allow him to avoid glitches.
 133
 134Locking
 135-------
 136
 137The PWM core list manipulations are protected by a mutex, so pwm_request()
 138and pwm_free() may not be called from an atomic context. Currently the
 139PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
 140pwm_config(), so the calling context is currently driver specific. This
 141is an issue derived from the former barebone API and should be fixed soon.
 142
 143Helpers
 144-------
 145
 146Currently a PWM can only be configured with period_ns and duty_ns. For several
 147use cases freq_hz and duty_percent might be better. Instead of calculating
 148this in your driver please consider adding appropriate helpers to the framework.
 149