From 0c2498f1660878339350bea8d18550b1b87ca055 Mon Sep 17 00:00:00 2001 From: Sascha Hauer Date: Fri, 28 Jan 2011 09:40:40 +0100 Subject: pwm: Add PWM framework support This patch adds framework support for PWM (pulse width modulation) devices. The is a barebone PWM API already in the kernel under include/linux/pwm.h, but it does not allow for multiple drivers as each of them implements the pwm_*() functions. There are other PWM framework patches around from Bill Gatliff. Unlike his framework this one does not change the existing API for PWMs so that this framework can act as a drop in replacement for the existing API. Why another framework? Several people argue that there should not be another framework for PWMs but they should be integrated into one of the existing frameworks like led or hwmon. Unlike these frameworks the PWM framework is agnostic to the purpose of the PWM. In fact, a PWM can drive a LED, but this makes the LED framework a user of a PWM, like already done in leds-pwm.c. The gpio framework also is not suitable for PWMs. Every gpio could be turned into a PWM using timer based toggling, but on the other hand not every PWM hardware device can be turned into a gpio due to the lack of hardware capabilities. This patch does not try to improve the PWM API yet, this could be done in subsequent patches. Signed-off-by: Sascha Hauer Acked-by: Kurt Van Dijck Reviewed-by: Arnd Bergmann Reviewed-by: Matthias Kaehlcke Reviewed-by: Mark Brown Reviewed-by: Shawn Guo [thierry.reding@avionic-design.de: fixup typos, kerneldoc comments] Signed-off-by: Thierry Reding --- Documentation/pwm.txt | 54 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 54 insertions(+) create mode 100644 Documentation/pwm.txt (limited to 'Documentation/pwm.txt') diff --git a/Documentation/pwm.txt b/Documentation/pwm.txt new file mode 100644 index 000000000000..03e39d145911 --- /dev/null +++ b/Documentation/pwm.txt @@ -0,0 +1,54 @@ +Pulse Width Modulation (PWM) interface + +This provides an overview about the Linux PWM interface + +PWMs are commonly used for controlling LEDs, fans or vibrators in +cell phones. PWMs with a fixed purpose have no need implementing +the Linux PWM API (although they could). However, PWMs are often +found as discrete devices on SoCs which have no fixed purpose. It's +up to the board designer to connect them to LEDs or fans. To provide +this kind of flexibility the generic PWM API exists. + +Identifying PWMs +---------------- + +Users of the legacy PWM API use unique IDs to refer to PWM devices. One +goal of the new PWM framework is to get rid of this global namespace. + +Using PWMs +---------- + +A PWM can be requested using pwm_request() and freed after usage with +pwm_free(). After being requested a PWM has to be configured using + +int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns); + +To start/stop toggling the PWM output use pwm_enable()/pwm_disable(). + +Implementing a PWM driver +------------------------- + +Currently there are two ways to implement pwm drivers. Traditionally +there only has been the barebone API meaning that each driver has +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 device 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 the ops and the pwm id to the framework. + +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 +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. + +Helpers +------- + +Currently a PWM can only be configured with period_ns and duty_ns. For several +use cases freq_hz and duty_percent might be better. Instead of calculating +this in your driver please consider adding appropriate helpers to the framework. -- cgit From f051c466cf690ac661d713d3ceb56b4efcecc853 Mon Sep 17 00:00:00 2001 From: Thierry Reding Date: Wed, 14 Dec 2011 11:12:23 +0100 Subject: pwm: Allow chips to support multiple PWMs MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Many PWM controllers provide access to more than a single PWM output and may even share some resource among them. Allowing a PWM chip to provide multiple PWM devices enables better sharing of those resources. As a side-effect this change allows easy integration with the device tree where a given PWM can be looked up based on the PWM chip's phandle and a corresponding index. This commit modifies the PWM core to support multiple PWMs per struct pwm_chip. It achieves this in a similar way to how gpiolib works, by allowing PWM ranges to be requested dynamically (pwm_chip.base == -1) or starting at a given offset (pwm_chip.base >= 0). A chip specifies how many PWMs it controls using the npwm member. Each of the functions in the pwm_ops structure gets an additional argument that specified the PWM number (it can be converted to a per-chip index by subtracting the chip's base). The total maximum number of PWM devices is currently fixed to 1024 while the data is actually stored in a radix tree, thus saving resources if not all of them are used. Reviewed-by: Mark Brown Reviewed-by: Shawn Guo [eric@eukrea.com: fix error handling in pwmchip_add] Signed-off-by: Eric Bénard Signed-off-by: Thierry Reding --- Documentation/pwm.txt | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) (limited to 'Documentation/pwm.txt') diff --git a/Documentation/pwm.txt b/Documentation/pwm.txt index 03e39d145911..48f598acdd16 100644 --- a/Documentation/pwm.txt +++ b/Documentation/pwm.txt @@ -33,9 +33,12 @@ there only has been the barebone API meaning that each driver has 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 device 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 the ops and the pwm id to the 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 provider by the chip and the chip-specific +implementation of the supported PWM operations to the framework. Locking ------- -- cgit From 8138d2ddbcca2a100482dac390133f83c5a60f94 Mon Sep 17 00:00:00 2001 From: Thierry Reding Date: Mon, 26 Mar 2012 08:42:48 +0200 Subject: pwm: Add table-based lookup for static mappings In order to get rid of the global namespace for PWM devices, this commit provides an alternative method, similar to that of the regulator or clock frameworks, for registering a static mapping for PWM devices. This works by providing a table with a provider/consumer map in the board setup code. With the new pwm_get() and pwm_put() functions available, usage of pwm_request() and pwm_free() becomes deprecated. Reviewed-by: Shawn Guo Reviewed-by: Mark Brown Signed-off-by: Thierry Reding --- Documentation/pwm.txt | 27 +++++++++++++++++++++++---- 1 file changed, 23 insertions(+), 4 deletions(-) (limited to 'Documentation/pwm.txt') diff --git a/Documentation/pwm.txt b/Documentation/pwm.txt index 48f598acdd16..554290ebab94 100644 --- a/Documentation/pwm.txt +++ b/Documentation/pwm.txt @@ -12,14 +12,33 @@ this kind of flexibility the generic PWM API exists. Identifying PWMs ---------------- -Users of the legacy PWM API use unique IDs to refer to PWM devices. One -goal of the new PWM framework is to get rid of this global namespace. +Users of the legacy PWM API use unique IDs to refer to PWM devices. + +Instead of referring to a PWM device via its unique ID, board setup code +should instead register a static mapping that can be used to match PWM +consumers to providers, as given in the following example: + + static struct pwm_lookup board_pwm_lookup[] = { + PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL), + }; + + static void __init board_init(void) + { + ... + pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup)); + ... + } Using PWMs ---------- -A PWM can be requested using pwm_request() and freed after usage with -pwm_free(). After being requested a PWM has to be configured using +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. + +After being requested a PWM has to be configured using: int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns); -- cgit