Documentation/pwm.txt

   1 Pulse Width Modulation (PWM) interface
   2
   3 This provides an overview about the Linux PWM interface
   4
   5 PWMs are commonly used for controlling LEDs, fans or vibrators in
   6 cell phones. PWMs with a fixed purpose have no need implementing
   7 the Linux PWM API (although they could). However, PWMs are often
   8 found as discrete devices on SoCs which have no fixed purpose. It's
   9 up to the board designer to connect them to LEDs or fans. To provide
  10 this kind of flexibility the generic PWM API exists.
  11
  12 Identifying PWMs
  13 ----------------
  14
  15 Users of the legacy PWM API use unique IDs to refer to PWM devices. One
  16 goal of the new PWM framework is to get rid of this global namespace.
  17
  18 Using PWMs
  19 ----------
  20
  21 A PWM can be requested using pwm_request() and freed after usage with
  22 pwm_free(). After being requested a PWM has to be configured using
  23
  24 int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
  25
  26 To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
  27
  28 Implementing a PWM driver
  29 -------------------------
  30
  31 Currently there are two ways to implement pwm drivers. Traditionally
  32 there only has been the barebone API meaning that each driver has
  33 to implement the pwm_*() functions itself. This means that it's impossible
  34 to have multiple PWM drivers in the system. For this reason it's mandatory
  35 for new drivers to use the generic PWM framework.
  36
  37 A new PWM controller/chip can be added using pwmchip_add() and removed
  38 again with pwmchip_remove(). pwmchip_add() takes a filled in struct
  39 pwm_chip as argument which provides a description of the PWM chip, the
  40 number of PWM devices provider by the chip and the chip-specific
  41 implementation of the supported PWM operations to the framework.
  42
  43 Locking
  44 -------
  45
  46 The PWM core list manipulations are protected by a mutex, so pwm_request()
  47 and pwm_free() may not be called from an atomic context. Currently the
  48 PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
  49 pwm_config(), so the calling context is currently driver specific. This
  50 is an issue derived from the former barebone API and should be fixed soon.
  51
  52 Helpers
  53 -------
  54
  55 Currently a PWM can only be configured with period_ns and duty_ns. For several
  56 use cases freq_hz and duty_percent might be better. Instead of calculating
  57 this in your driver please consider adding appropriate helpers to the framework.