[PATCH v12 02/13] doc: Add doc for the Ingenic TCU hardware

[Paul Cercueil]

Add a documentation file 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@xxxxxxxxxxxxxxx>
---

Notes:
    v4: New patch in this series
    
    v5: Added information about number of channels, and improved
     documentation about channel modes
    
    v6: Add info about OST (can be 32-bit on older SoCs)
    
    v7-v11: No change
    
    v12: Add details about new implementation

 Documentation/mips/ingenic-tcu.txt | 63 ++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)
 create mode 100644 Documentation/mips/ingenic-tcu.txt

diff --git a/Documentation/mips/ingenic-tcu.txt b/Documentation/mips/ingenic-tcu.txt
new file mode 100644
index 000000000000..1a753805779c
--- /dev/null
+++ b/Documentation/mips/ingenic-tcu.txt
@@ -0,0 +1,63 @@
+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 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:
+- boilerplate:      drivers/mfd/ingenic-tcu.c
+- clocks:           drivers/clk/ingenic/tcu.c
+- interrupts:       drivers/irqchip/irq-ingenic-tcu.c
+- timers:           drivers/clocksource/ingenic-timer.c
+- PWM:              drivers/pwm/pwm-jz4740.c
+- watchdog:         drivers/watchdog/jz4740_wdt.c
+- OST:              drivers/clocksource/ingenic-ost.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/mfd/ingenic,tcu.txt.
-- 
2.21.0.593.g511ec345e18