Re: [PATCH v2 2/2] hwmon: add initial support for AMD PROM21 xHCI temperature sensor

[Michal Pecio <michal.pecio@gmail.com>]

On Thu, 7 May 2026 07:41:28 +0900, Jihong Min wrote:
> > I think this commit message and certainly the Kconfig help text
> > should include full name of the chip and perhaps its official
> > marketing names too, so that people better understand what hardware
> > is supported.
> >
> > So: "AMD Promontory 21 chipset" and "AM5 6xx/8xx series chipsets",
> > or whatever they are called by AMD and motherboard vendors.  
> 
> Addressed locally for v3. I changed the commit message, Kconfig
> prompt/help text, and hwmon documentation to use the full name, "AMD
> Promontory 21 (PROM21)".
> 
> I avoided putting chipset marketing names directly into the Kconfig
> text and commit subject because some AMD 6xx/8xx series chipsets can
> be built from more than one PROM21 IP in a daisy-chained
> configuration, including more than one PROM21 xHCI controller.

Not sure how is this a problem?
The driver is still applicable to those daisy-chained chipsets.

Configuring the kernel is tedious enough already, the help text should
ideally tell me whether I need this driver or not without searching
for obscure code names.

I would even go as far as to mention that it's for AM5 chipsets,
because AMD recycles chipset numbers every few generations.

>  > Is there any documentation of the index/data registers themselves?
> 
> I am not aware of public AMD documentation for the PROM21 vendor
> index/data registers or the temperature selector.
> 
> For my initial validation on an X870E system with two decade or
> so.PROM21 xHCI controllers, I passed one PROM21 xHCI controller
> through to a Windows VM, left USB traffic idle, and let a Windows
> monitoring utility poll the controller temperature. From the Linux
> host I monitored that controller's PCI MMIO BAR with read-only
> accesses.
>
> The vendor index register repeatedly held the same selector value,
> and the adjacent data register exposed a stable low 8-bit value.
> [...]
>
> After identifying the register pair, I used the same validation setup
> to derive the conversion formula by comparing the observed raw MMIO
> register value against HWiNFO64's reported PROM21 xHCI temperature on
> Windows.

Looks good, unless there are gotchas like the actual formula being
dependent on other factors you haven't noticed HWiNFO reading.

It would be good idea to record this methodology in the commit message
or docs so that others can repeat your experiment in case of issues.

Regards,
Michal