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>

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.