hwmon: (kb9002) Add documentation

Document the sysfs and debugfs interfaces of the Kandou KB9002 hwmon
driver.

Signed-off-by: Andy Chung <Andy.Chung@amd.com>
Link: https://lore.kernel.org/r/20260724-kb9002-upstream-v5-4-9523c3ffe83a@amd.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/index.rst

@@ -112,6 +112,7 @@ Hardware Monitoring Kernel Drivers
    jc42
    k10temp
    k8temp
+   kb9002
    kbatt
    kfan
    lan966x

Documentation/hwmon/kb9002.rst

@@ -0,0 +1,64 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Kernel driver kb9002
+====================
+
+Supported chips:
+
+  * Kandou KB9002
+
+    Prefix: 'kb9002'
+
+    Addresses scanned: -
+
+    Datasheet: KA-015171-PD (available from Kandou under NDA)
+
+Author: Andy Chung <andy.chung@amd.com>
+
+Description
+-----------
+
+The Kandou KB9002 is an 8-lane PCIe 5.0 retimer with an integrated
+microcontroller. It exposes an SMBus 3.0 target (with mandatory PEC)
+on its sideband interface. The internal firmware aggregates per-lane
+die temperatures and publishes the maximum value through a 16-bit
+addressed register window.
+
+This driver reports that aggregated maximum as the only hwmon
+temperature channel. The running firmware version and the firmware
+boot status are exposed under debugfs.
+
+sysfs interface
+---------------
+
+==================  ===============================================
+temp1_input         Aggregated maximum die temperature across all
+                    active lanes (millidegrees Celsius).
+==================  ===============================================
+
+debugfs interface
+-----------------
+
+Files live in the per-client debugfs directory created by the I2C
+core: ``/sys/kernel/debug/i2c/i2c-<bus>/<bus>-<addr>/``.
+
+==================  ===============================================
+fw_ver              Running firmware version in
+                    "major.minor.patch.suffix" format. Read-only.
+fw_load_status      Firmware boot status: "normal" once firmware has
+                    finished initialising, "abnormal" otherwise.
+                    Read-only.
+==================  ===============================================
+
+Notes
+-----
+
+The driver requires ``I2C_FUNC_SMBUS_BLOCK_DATA``,
+``I2C_FUNC_SMBUS_PEC`` and ``I2C_FUNC_I2C`` from the host adapter. The
+last is needed only during probe for the host-interface mode switch;
+runtime accesses use SMBus block transactions exclusively.
+
+The retimer's SMBus address is configurable on three strap pins and
+ranges from 0x20 to 0x27. The address is selected through device tree
+(or i2c board info) the same way as any other I2C device; the driver
+does not auto-detect.