.. SPDX-License-Identifier: GPL-2.0

===============================================
Ingenic JZ47xx SoCs Timer/Counter Unit hardware
===============================================

The Timer/Counter Unit (TCU) in Ingenic JZ47xx SoCs is a multi-function
hardware block. It features up 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 JZ4760B 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.

Implementation
==============

The functionalities of the TCU hardware are spread across multiple drivers:

===========  =====
clocks       drivers/clk/ingenic/tcu.c
interrupts   drivers/irqchip/irq-ingenic-tcu.c
timers       drivers/clocksource/ingenic-timer.c
OST          drivers/clocksource/ingenic-ost.c
PWM          drivers/pwm/pwm-jz4740.c
watchdog     drivers/watchdog/jz4740_wdt.c
===========  =====

Because various functionalities of the TCU that belong to different drivers
and frameworks can be controlled from the same registers, all of these
drivers access their registers through the same regmap.

For more information regarding the devicetree bindings of the TCU drivers,
have a look at Documentation/devicetree/bindings/timer/ingenic,tcu.yaml.
