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doc: Add doc for the Ingenic TCU hardware
Add documentation about the Timer/Counter Unit (TCU) present in the
Ingenic JZ47xx SoCs.
The Timer/Counter Unit (TCU) in Ingenic JZ47xx SoCs is a multi-function
hardware block. It features up to to eight channels, that can be used as
counters, timers, or PWM.
- JZ4725B, JZ4750, JZ4755 only have six TCU channels. The other SoCs all
have eight channels.
- JZ4725B introduced a separate channel, called Operating System Timer
(OST). It is a 32-bit programmable timer. On JZ4770 and above, it is
64-bit.
- Each one of the TCU channels has its own clock, which can be reparented
to three different clocks (pclk, ext, rtc), gated, and reclocked, through
their TCSR register.
* The watchdog and OST hardware blocks also feature a TCSR register with
the same format in their register space.
* The TCU registers used to gate/ungate can also gate/ungate the watchdog
and OST clocks.
- Each TCU channel works in one of two modes:
* mode TCU1: channels cannot work in sleep mode, but are easier to
operate.
* mode TCU2: channels can work in sleep mode, but the operation is a bit
more complicated than with TCU1 channels.
- The mode of each TCU channel depends on the SoC used:
* On the oldest SoCs (up to JZ4740), all of the eight channels operate in
TCU1 mode.
* On JZ4725B, channel 5 operates as TCU2, the others operate as TCU1.
* On newest SoCs (JZ4750 and above), channels 1-2 operate as TCU2, the
others operate as TCU1.
- Each channel can generate an interrupt. Some channels share an interrupt
line, some don't, and this changes between SoC versions:
* on older SoCs (JZ4740 and below), channel 0 and channel 1 have their
own interrupt line; channels 2-7 share the last interrupt line.
* On JZ4725B, channel 0 has its own interrupt; channels 1-5 share one
interrupt line; the OST uses the last interrupt line.
* on newer SoCs (JZ4750 and above), channel 5 has its own interrupt;
channels 0-4 and (if eight channels) 6-7 all share one interrupt line;
the OST uses the last interrupt line.
Signed-off-by: 
Paul Cercueil <paul@crapouillou.net>
Tested-by: Mathieu Malaterre <malat@debian.org>
Tested-by: Artur Rojek <contact@artur-rojek.eu>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-mips@vger.kernel.org
Cc: linux-clk@vger.kernel.org
Cc: od@zcrc.me