linux/Documentation/pwm.txt
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   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        };
  24
  25        static void __init board_init(void)
  26        {
  27                ...
  28                pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
  29                ...
  30        }
  31
  32Using PWMs
  33----------
  34
  35Legacy users can request a PWM device using pwm_request() and free it
  36after usage with pwm_free().
  37
  38New users should use the pwm_get() function and pass to it the consumer
  39device or a consumer name. pwm_put() is used to free the PWM device. Managed
  40variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
  41
  42After being requested a PWM has to be configured using:
  43
  44int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
  45
  46To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
  47
  48Implementing a PWM driver
  49-------------------------
  50
  51Currently there are two ways to implement pwm drivers. Traditionally
  52there only has been the barebone API meaning that each driver has
  53to implement the pwm_*() functions itself. This means that it's impossible
  54to have multiple PWM drivers in the system. For this reason it's mandatory
  55for new drivers to use the generic PWM framework.
  56
  57A new PWM controller/chip can be added using pwmchip_add() and removed
  58again with pwmchip_remove(). pwmchip_add() takes a filled in struct
  59pwm_chip as argument which provides a description of the PWM chip, the
  60number of PWM devices provider by the chip and the chip-specific
  61implementation of the supported PWM operations to the framework.
  62
  63Locking
  64-------
  65
  66The PWM core list manipulations are protected by a mutex, so pwm_request()
  67and pwm_free() may not be called from an atomic context. Currently the
  68PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
  69pwm_config(), so the calling context is currently driver specific. This
  70is an issue derived from the former barebone API and should be fixed soon.
  71
  72Helpers
  73-------
  74
  75Currently a PWM can only be configured with period_ns and duty_ns. For several
  76use cases freq_hz and duty_percent might be better. Instead of calculating
  77this in your driver please consider adding appropriate helpers to the framework.
  78
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