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pwm: meson: Add documentation to the driver

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Add links to the datasheet and a short summary how the hardware works.
The goal is to make it easier for other developers to understand why the
pwm-meson driver is implemented the way it is.

Suggested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Co-authored-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
This commit is contained in:
Martin Blumenstingl 2019-06-12 21:59:11 +02:00 committed by Thierry Reding
parent 7341c785d8
commit 4ae42ce793
1 changed files with 22 additions and 0 deletions
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22
drivers/pwm/pwm-meson.c
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@ -1,5 +1,27 @@
// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
/*
* PWM controller driver for Amlogic Meson SoCs.
*
* This PWM is only a set of Gates, Dividers and Counters:
* PWM output is achieved by calculating a clock that permits calculating
* two periods (low and high). The counter then has to be set to switch after
* N cycles for the first half period.
* The hardware has no "polarity" setting. This driver reverses the period
* cycles (the low length is inverted with the high length) for
* PWM_POLARITY_INVERSED. This means that .get_state cannot read the polarity
* from the hardware.
* Setting the duty cycle will disable and re-enable the PWM output.
* Disabling the PWM stops the output immediately (without waiting for the
* current period to complete first).
*
* The public S912 (GXM) datasheet contains some documentation for this PWM
* controller starting on page 543:
* https://dl.khadas.com/Hardware/VIM2/Datasheet/S912_Datasheet_V0.220170314publicversion-Wesion.pdf
* An updated version of this IP block is found in S922X (G12B) SoCs. The
* datasheet contains the description for this IP block revision starting at
* page 1084:
* https://dn.odroid.com/S922X/ODROID-N2/Datasheet/S922X_Public_Datasheet_V0.2.pdf
*
* Copyright (c) 2016 BayLibre, SAS.
* Author: Neil Armstrong <narmstrong@baylibre.com>
* Copyright (C) 2014 Amlogic, Inc.