linux-kernel - [PATCH v5 22/24] pwm: update documentation

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Date:	Thu, 14 Apr 2016 21:17:42 +0200
From:	Boris Brezillon <boris.brezillon@...e-electrons.com>
To:	Thierry Reding <thierry.reding@...il.com>,
	linux-pwm@...r.kernel.org
Cc:	Mike Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>, linux-clk@...r.kernel.org,
	Mark Brown <broonie@...nel.org>,
	Liam Girdwood <lgirdwood@...il.com>,
	Kamil Debski <k.debski@...sung.com>, lm-sensors@...sensors.org,
	Jean Delvare <jdelvare@...e.com>,
	Guenter Roeck <linux@...ck-us.net>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	linux-input@...r.kernel.org, Bryan Wu <cooloney@...il.com>,
	Richard Purdie <rpurdie@...ys.net>,
	Jacek Anaszewski <j.anaszewski@...sung.com>,
	linux-leds@...r.kernel.org,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Chen-Yu Tsai <wens@...e.org>, linux-sunxi@...glegroups.com,
	Joachim Eastwood <manabian@...il.com>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Heiko Stuebner <heiko@...ech.de>,
	linux-rockchip@...ts.infradead.org,
	Jingoo Han <jingoohan1@...il.com>,
	Lee Jones <lee.jones@...aro.org>, linux-fbdev@...r.kernel.org,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	Robert Jarzmik <robert.jarzmik@...e.fr>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Kukjin Kim <kgene@...nel.org>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	linux-samsung-soc@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
	Daniel Vetter <daniel.vetter@...el.com>,
	Jani Nikula <jani.nikula@...ux.intel.com>,
	Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Hartley Sweeten <hsweeten@...ionengravers.com>,
	Ryan Mallon <rmallon@...il.com>,
	Alexander Shiyan <shc_work@...l.ru>,
	Milo Kim <milo.kim@...com>,
	Doug Anderson <dianders@...gle.com>,
	Caesar Wang <wxt@...k-chips.com>,
	Stephen Barber <smbarber@...omium.org>,
	Boris Brezillon <boris.brezillon@...e-electrons.com>
Subject: [PATCH v5 22/24] pwm: update documentation

Update the PWM subsystem documentation to reflect the atomic PWM changes.

Signed-off-by: Boris Brezillon <boris.brezillon@...e-electrons.com>
---
 Documentation/pwm.txt | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/Documentation/pwm.txt b/Documentation/pwm.txt
index ca895fd..789b27c 100644
--- a/Documentation/pwm.txt
+++ b/Documentation/pwm.txt
@@ -42,9 +42,26 @@ variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
 
 After being requested, a PWM has to be configured using:
 
-int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
+int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state);
 
-To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
+This API controls both the PWM period/duty_cycle config and the
+enable/disable state.
+
+The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
+around pwm_apply_state() and should not be used if the user wants to change
+several parameter at once. For example, if you see pwm_config() and
+pwm_{enable,disable}() calls in the same function, this probably means you
+should switch to pwm_apply_state().
+
+The PWM user API also allows one to query the PWM state with pwm_get_state().
+
+In addition to the PWM state, the PWM API also exposes PWM arguments, which
+are the reference PWM config one should use on this PWM.
+PWM arguments are usually platform-specific and allows the PWM user to only
+care about dutycycle relatively to the full period (like, duty = 50% of the
+period). struct pwm_args contains 2 fields (period and polarity) and should
+be used to set the initial PWM config (usually done in the probe function
+of the PWM user). PWM arguments are retrieved with pwm_get_args().
 
 Using PWMs with the sysfs interface
 -----------------------------------
@@ -105,6 +122,15 @@ goes low for the remainder of the period. Conversely, a signal with inversed
 polarity starts low for the duration of the duty cycle and goes high for the
 remainder of the period.
 
+Drivers are encouraged to implement ->apply() instead of the legacy
+->enable(), ->disable() and ->config() methods. Doing that should provide
+atomicity in the PWM config workflow, which is required when the PWM controls
+a critical device (like a regulator).
+
+The implementation of ->get_state() (a method used to retrieve initial PWM
+state) is also encouraged for the same reason: letting the PWM user know
+about the current PWM state would allow him to avoid glitches.
+
 Locking
 -------
 
-- 
2.5.0