[PATCH 4/4] docs/system/riscv: document K230 CMU support

Signed-off-by: Simon Law <[email protected]>
---
 docs/system/riscv/k230.rst | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/docs/system/riscv/k230.rst b/docs/system/riscv/k230.rst
index cea8202e55..dd50b0bd2e 100644
--- a/docs/system/riscv/k230.rst
+++ b/docs/system/riscv/k230.rst
@@ -18,9 +18,31 @@ The ``k230`` machine supports the following devices:
 * 1 c908 cores (little core)
 * Core Local Interruptor (CLINT)
 * Platform-Level Interrupt Controller (PLIC)
+* Clock Management Unit (CMU)
 * 2 K230 Watchdog Timer
 * 5 UART
 
+Clock Management Unit
+---------------------
+
+The CMU model provides the register interface used by the K230 SDK U-Boot and
+the upstream Linux K230 clock driver. It implements the four PLL register
+blocks at ``0x91102000`` through ``0x9110203f`` and the defined sysclk
+registers in the ``0x91100000`` window, including the software-visible
+``SPI2AXI_CLK_DIV`` register at offset ``0x108``.
+
+The model implements reset values, writable and reserved bits, PLL control
+write-enable semantics, and write-one-to-trigger update operations. PLL lock
+and init completion are deterministic and immediate so that Linux clock probe
+does not depend on virtual time.
+
+The CMU does not propagate clock frequencies to QEMU devices or alter CPU
+execution speed. Peripheral reset, power control, and a guest-initiated K230
+system-reset backend are separate hardware blocks and are not modeled by the
+CMU. Consequently, a guest reboot requires the future K230 reset subsystem
+and matching OpenSBI reboot backend; QMP ``system_reset`` resets the CMU along
+with the rest of the machine.
+
 Boot options
 ------------
 The ``k230`` machine supports K230 SDK boot through M-mode U-Boot, which then

-- 
2.43.0