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Mauro Carvalho Chehabc5711232017-06-17 12:26:44 -03001======================================
Sascha Hauer0c2498f2011-01-28 09:40:40 +01002Pulse Width Modulation (PWM) interface
Mauro Carvalho Chehabc5711232017-06-17 12:26:44 -03003======================================
Sascha Hauer0c2498f2011-01-28 09:40:40 +01004
5This provides an overview about the Linux PWM interface
6
7PWMs are commonly used for controlling LEDs, fans or vibrators in
8cell phones. PWMs with a fixed purpose have no need implementing
9the Linux PWM API (although they could). However, PWMs are often
10found as discrete devices on SoCs which have no fixed purpose. It's
11up to the board designer to connect them to LEDs or fans. To provide
12this kind of flexibility the generic PWM API exists.
13
14Identifying PWMs
15----------------
16
Thierry Reding8138d2d2012-03-26 08:42:48 +020017Users of the legacy PWM API use unique IDs to refer to PWM devices.
18
19Instead of referring to a PWM device via its unique ID, board setup code
20should instead register a static mapping that can be used to match PWM
Mauro Carvalho Chehabc5711232017-06-17 12:26:44 -030021consumers to providers, as given in the following example::
Thierry Reding8138d2d2012-03-26 08:42:48 +020022
23 static struct pwm_lookup board_pwm_lookup[] = {
Alexandre Belloni42844022014-05-19 22:42:37 +020024 PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL,
25 50000, PWM_POLARITY_NORMAL),
Thierry Reding8138d2d2012-03-26 08:42:48 +020026 };
27
28 static void __init board_init(void)
29 {
30 ...
31 pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
32 ...
33 }
Sascha Hauer0c2498f2011-01-28 09:40:40 +010034
35Using PWMs
36----------
37
Thierry Reding8138d2d2012-03-26 08:42:48 +020038Legacy users can request a PWM device using pwm_request() and free it
39after usage with pwm_free().
40
41New users should use the pwm_get() function and pass to it the consumer
Alexandre Courbot63543162012-08-01 19:20:58 +090042device or a consumer name. pwm_put() is used to free the PWM device. Managed
43variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
Thierry Reding8138d2d2012-03-26 08:42:48 +020044
Mauro Carvalho Chehabc5711232017-06-17 12:26:44 -030045After being requested, a PWM has to be configured using::
Sascha Hauer0c2498f2011-01-28 09:40:40 +010046
Mauro Carvalho Chehabc5711232017-06-17 12:26:44 -030047 int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state);
Sascha Hauer0c2498f2011-01-28 09:40:40 +010048
Boris Brezillona07136f2016-04-14 21:17:42 +020049This API controls both the PWM period/duty_cycle config and the
50enable/disable state.
51
52The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
53around pwm_apply_state() and should not be used if the user wants to change
54several parameter at once. For example, if you see pwm_config() and
55pwm_{enable,disable}() calls in the same function, this probably means you
56should switch to pwm_apply_state().
57
58The PWM user API also allows one to query the PWM state with pwm_get_state().
59
60In addition to the PWM state, the PWM API also exposes PWM arguments, which
61are the reference PWM config one should use on this PWM.
62PWM arguments are usually platform-specific and allows the PWM user to only
63care about dutycycle relatively to the full period (like, duty = 50% of the
64period). struct pwm_args contains 2 fields (period and polarity) and should
65be used to set the initial PWM config (usually done in the probe function
66of the PWM user). PWM arguments are retrieved with pwm_get_args().
Sascha Hauer0c2498f2011-01-28 09:40:40 +010067
Yoshihiro Shimoda321a7ce2019-05-31 18:54:58 +090068All consumers should really be reconfiguring the PWM upon resume as
69appropriate. This is the only way to ensure that everything is resumed in
70the proper order.
71
H Hartley Sweeten76abbdde2013-06-11 10:38:59 -070072Using PWMs with the sysfs interface
73-----------------------------------
74
75If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
76interface is provided to use the PWMs from userspace. It is exposed at
77/sys/class/pwm/. Each probed PWM controller/chip will be exported as
78pwmchipN, where N is the base of the PWM chip. Inside the directory you
79will find:
80
Mauro Carvalho Chehabc5711232017-06-17 12:26:44 -030081 npwm
82 The number of PWM channels this chip supports (read-only).
H Hartley Sweeten76abbdde2013-06-11 10:38:59 -070083
Mauro Carvalho Chehabc5711232017-06-17 12:26:44 -030084 export
85 Exports a PWM channel for use with sysfs (write-only).
H Hartley Sweeten76abbdde2013-06-11 10:38:59 -070086
Mauro Carvalho Chehabc5711232017-06-17 12:26:44 -030087 unexport
88 Unexports a PWM channel from sysfs (write-only).
H Hartley Sweeten76abbdde2013-06-11 10:38:59 -070089
90The PWM channels are numbered using a per-chip index from 0 to npwm-1.
91
92When a PWM channel is exported a pwmX directory will be created in the
93pwmchipN directory it is associated with, where X is the number of the
94channel that was exported. The following properties will then be available:
95
Mauro Carvalho Chehabc5711232017-06-17 12:26:44 -030096 period
97 The total period of the PWM signal (read/write).
98 Value is in nanoseconds and is the sum of the active and inactive
99 time of the PWM.
H Hartley Sweeten76abbdde2013-06-11 10:38:59 -0700100
Mauro Carvalho Chehabc5711232017-06-17 12:26:44 -0300101 duty_cycle
102 The active time of the PWM signal (read/write).
103 Value is in nanoseconds and must be less than the period.
H Hartley Sweeten76abbdde2013-06-11 10:38:59 -0700104
Mauro Carvalho Chehabc5711232017-06-17 12:26:44 -0300105 polarity
106 Changes the polarity of the PWM signal (read/write).
107 Writes to this property only work if the PWM chip supports changing
108 the polarity. The polarity can only be changed if the PWM is not
109 enabled. Value is the string "normal" or "inversed".
H Hartley Sweeten76abbdde2013-06-11 10:38:59 -0700110
Mauro Carvalho Chehabc5711232017-06-17 12:26:44 -0300111 enable
112 Enable/disable the PWM signal (read/write).
113
114 - 0 - disabled
115 - 1 - enabled
H Hartley Sweeten76abbdde2013-06-11 10:38:59 -0700116
Sascha Hauer0c2498f2011-01-28 09:40:40 +0100117Implementing a PWM driver
118-------------------------
119
120Currently there are two ways to implement pwm drivers. Traditionally
121there only has been the barebone API meaning that each driver has
122to implement the pwm_*() functions itself. This means that it's impossible
123to have multiple PWM drivers in the system. For this reason it's mandatory
124for new drivers to use the generic PWM framework.
Thierry Redingf051c462011-12-14 11:12:23 +0100125
126A new PWM controller/chip can be added using pwmchip_add() and removed
127again with pwmchip_remove(). pwmchip_add() takes a filled in struct
128pwm_chip as argument which provides a description of the PWM chip, the
Sachin Kamat702e3042013-10-24 14:13:47 +0530129number of PWM devices provided by the chip and the chip-specific
Thierry Redingf051c462011-12-14 11:12:23 +0100130implementation of the supported PWM operations to the framework.
Sascha Hauer0c2498f2011-01-28 09:40:40 +0100131
Thierry Reding3e5314d2014-04-07 15:45:30 +0200132When implementing polarity support in a PWM driver, make sure to respect the
133signal conventions in the PWM framework. By definition, normal polarity
134characterizes a signal starts high for the duration of the duty cycle and
135goes low for the remainder of the period. Conversely, a signal with inversed
136polarity starts low for the duration of the duty cycle and goes high for the
137remainder of the period.
138
Boris Brezillona07136f2016-04-14 21:17:42 +0200139Drivers are encouraged to implement ->apply() instead of the legacy
140->enable(), ->disable() and ->config() methods. Doing that should provide
141atomicity in the PWM config workflow, which is required when the PWM controls
142a critical device (like a regulator).
143
144The implementation of ->get_state() (a method used to retrieve initial PWM
145state) is also encouraged for the same reason: letting the PWM user know
146about the current PWM state would allow him to avoid glitches.
147
Yoshihiro Shimoda321a7ce2019-05-31 18:54:58 +0900148Drivers should not implement any power management. In other words,
149consumers should implement it as described in the "Using PWMs" section.
150
Sascha Hauer0c2498f2011-01-28 09:40:40 +0100151Locking
152-------
153
154The PWM core list manipulations are protected by a mutex, so pwm_request()
155and pwm_free() may not be called from an atomic context. Currently the
156PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
157pwm_config(), so the calling context is currently driver specific. This
158is an issue derived from the former barebone API and should be fixed soon.
159
160Helpers
161-------
162
163Currently a PWM can only be configured with period_ns and duty_ns. For several
164use cases freq_hz and duty_percent might be better. Instead of calculating
165this in your driver please consider adding appropriate helpers to the framework.