hwmon: add AMD Promontory 21 xHCI temperature sensor support

Add an auxiliary-bus hwmon driver for the temperature sensor exposed by
AMD Promontory 21 (PROM21) xHCI PCI functions. The driver binds to the
"hwmon" auxiliary device published by the PROM21 xHCI PCI glue and
exposes the sensor as temp1_input under the prom21_xhci hwmon device.

The sensor is accessed through a PROM21 vendor index/data register pair
in the xHCI PCI MMIO BAR. The driver consumes parent-provided MMIO data
from the PROM21 PCI glue instead of inspecting the parent PCI driver's
drvdata. The read path restores the previous vendor index value after
sampling and does not runtime-resume the parent PCI device; reads from a
suspended parent return -ENODATA.

Document the supported device, register access, runtime PM behavior, and
sysfs lookup method. The documentation also records the observation
method used to identify the register pair and derive the conversion
formula.

Assisted-by: Codex:gpt-5.5
Signed-off-by: Jihong Min <hurryman2212@gmail.com>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Tested-by: Yaroslav Isakov <yaroslav.isakov@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://patch.msgid.link/20260519000732.2334711-3-hurryman2212@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

0 files changed, 0 insertions, 0 deletions