Hi,

This series adds hwmon support for the temperature sensor exposed by AMD
PROM21 xHCI controllers. The temperature value is read through a
vendor-specific index/data register pair in the xHCI PCI MMIO BAR.

v1 implemented this directly under drivers/usb/host. Based on review
feedback, v2 splits the support into a generic xhci-pci auxiliary-device
interface and a standalone hwmon driver under drivers/hwmon. xhci-pci
creates the auxiliary child only for PCI IDs matched by its internal table,
currently only AMD 1022:43fd, and the hwmon driver binds to that child
through the auxiliary bus.

The PROM21 hwmon driver registers the hwmon device under the parent PCI
function so userspace reports it as a PCI adapter. The auxiliary driver
still owns the lifetime and unregisters the hwmon device before xhci-pci
tears down the HCD.

I tested register access while the parent xHCI PCI function was suspended.
The temperature register did not return a valid value without
runtime-resuming the device. v2 therefore keeps the default behavior of
allowing a temperature read to wake the xHCI PCI device, and adds
allow_pm_switch=N for users who prefer reads not to wake the device. In
that mode a read returns -EAGAIN while the device is suspended.

Changes in v2:
- split the original single patch into a two-patch series
- add a generic xhci-pci auxiliary-device interface
- move PROM21 hwmon support to drivers/hwmon as an auxiliary driver
- avoid linking hwmon-specific code into xhci-hcd
- add Documentation/hwmon/prom21-hwmon.rst
- document register offsets, conversion formula, sysfs attributes, and
  allow_pm_switch behavior

Sincerely,
Jihong Min

Jihong Min (2):
  usb: xhci-pci: add generic auxiliary device interface
  hwmon: add initial support for AMD PROM21 xHCI temperature sensor

 Documentation/hwmon/index.rst        |   1 +
 Documentation/hwmon/prom21-hwmon.rst |  84 ++++++++
 drivers/hwmon/Kconfig                |  11 +
 drivers/hwmon/Makefile               |   1 +
 drivers/hwmon/prom21-hwmon.c         | 300 +++++++++++++++++++++++++++
 drivers/usb/host/Kconfig             |  10 +
 drivers/usb/host/xhci-pci.c          | 114 ++++++++++
 7 files changed, 521 insertions(+)
 create mode 100644 Documentation/hwmon/prom21-hwmon.rst
 create mode 100644 drivers/hwmon/prom21-hwmon.c

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2.53.0