[PATCH v2 2/3] docs: watchdog: npcm: Add reset status description

[Tomer Maimon <tmaimon77@gmail.com>]

Add documentation describing how the NPCM watchdog driver reports reset
causes through bootstatus on NPCM750 and NPCM845 systems.

Document the reset flag mapping, the watchdog instance mapping for
WDIOF_CARDRESET, and the NPCM750/NPCM845 latch handling. Also mention
sysfs bootstatus reporting when watchdog sysfs support is enabled.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
---
 Documentation/watchdog/index.rst    |  1 +
 Documentation/watchdog/npcm_wdt.rst | 70 +++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+)
 create mode 100644 Documentation/watchdog/npcm_wdt.rst

diff --git a/Documentation/watchdog/index.rst b/Documentation/watchdog/index.rst
index 1cea24681e6b..ef29e861e837 100644
--- a/Documentation/watchdog/index.rst
+++ b/Documentation/watchdog/index.rst
@@ -9,6 +9,7 @@ Watchdog Support
 
     hpwdt
     mlx-wdt
+    npcm_wdt
     pcwd-watchdog
     watchdog-api
     watchdog-kernel-api
diff --git a/Documentation/watchdog/npcm_wdt.rst b/Documentation/watchdog/npcm_wdt.rst
new file mode 100644
index 000000000000..48f0c7920c11
--- /dev/null
+++ b/Documentation/watchdog/npcm_wdt.rst
@@ -0,0 +1,70 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+NPCM Watchdog
+=============
+
+The NPCM watchdog driver can report reset-cause information on
+``nuvoton,npcm750-wdt`` and ``nuvoton,npcm845-wdt`` systems.
+
+Userspace can read the latched reset cause through
+``WDIOC_GETBOOTSTATUS``. When ``CONFIG_WATCHDOG_SYSFS`` is enabled, the
+same value is also visible through ``/sys/class/watchdog/watchdogN/bootstatus``.
+
+The mapping is fixed in the driver. It exposes the SoC reset indications
+through the generic watchdog bootstatus flags and is not configurable from
+Device Tree.
+
+.. list-table:: Reset-cause mapping
+   :header-rows: 1
+
+   * - Platform
+     - Reset indication
+     - Bootstatus flag
+     - Reported meaning
+   * - NPCM750 and NPCM845
+     - ``PORST``
+     - ``WDIOF_OVERHEAT``
+     - power-on reset
+   * - NPCM750 and NPCM845
+     - ``CORST``
+     - ``WDIOF_FANFAULT``
+     - core reset
+   * - NPCM750 and NPCM845
+     - ``SWR1RST``
+     - ``WDIOF_EXTERN1``
+     - software reset source 1
+   * - NPCM750 and NPCM845
+     - ``SWR2RST``
+     - ``WDIOF_EXTERN2``
+     - software reset source 2
+   * - NPCM750 and NPCM845
+     - ``SWR3RST``
+     - ``WDIOF_POWERUNDER``
+     - software reset source 3
+   * - NPCM750
+     - ``SWR4RST``
+     - ``WDIOF_POWEROVER``
+     - software reset source 4
+   * - NPCM845
+     - ``TIP reset`` (``INTCR2[25]``)
+     - ``WDIOF_POWEROVER``
+     - TIP reset
+
+``WDIOF_CARDRESET`` is reported only for the watchdog instance whose own
+reset-status bit is latched. On systems with three watchdog instances, this
+maps ``WD0RST``, ``WD1RST``, and ``WD2RST`` to ``watchdog0``, ``watchdog1``,
+and ``watchdog2`` respectively.
+
+The driver may report ``WDIOF_CARDRESET`` together with one or more of the
+reset-cause flags listed above.
+
+On NPCM750, the driver samples ``RESSR`` and clears the latched reset bits
+after reading them.
+
+On NPCM845, the driver samples ``INTCR2``. When reset bits are still latched,
+it clears them and stores the sampled value in ``SCRPAD10`` so later watchdog
+probes can report the same boot-time state.
+
+The WPCM450 watchdog continues to operate without this reset-indication
+mapping.
-- 
2.34.1