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authorMartin Blumenstingl <martin.blumenstingl@googlemail.com>2019-06-12 21:59:11 +0200
committerThierry Reding <thierry.reding@gmail.com>2019-06-26 11:39:10 +0200
commit4ae42ce793a755decb2c5978623e56edfdf3ab39 (patch)
tree1ae13ccea70ffa4812a9b003d1bbdf7b3cf3f56b /drivers/pwm
parent7341c785d81ebf5472462b7f8ee27a96a59d5cf4 (diff)
downloadlinux-4ae42ce793a755decb2c5978623e56edfdf3ab39.tar.bz2
pwm: meson: Add documentation to the driver
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>
Diffstat (limited to 'drivers/pwm')
-rw-r--r--drivers/pwm/pwm-meson.c22
1 files changed, 22 insertions, 0 deletions
diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c
index f934828aaff9..3cbff5cbb789 100644
--- a/drivers/pwm/pwm-meson.c
+++ b/drivers/pwm/pwm-meson.c
@@ -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.