Re: [PATCH Resend] PCI: Limit islolated function probing on bus 0 for LoongArch

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Oct 14, 2025 at 03:41:00PM +0800, Huacai Chen wrote:
> We found some discrete AMD graphics devices hide funtion 0 and the whole
> is not supposed to be probed.
> 
> Since our original purpose is to allow integrated devices (on bus 0) to
> be probed without function 0, we can limit the islolated function probing
> only on bus 0.

s/islolated/isolated/ (multiple)
s/funtion/function/

I suppose this fixes some problem where:

  - An AMD GPU is on some bus other than 00
  - The GPU has no function 0
  - Without function 0, we normally don't probe other functions
  - a02fd05661d7 means we *do* probe other functions on LoongArch
  - Therefore we find some non-0 function we're not supposed to find

If that's the case, what bad thing happens?  Is there some dmesg hint
we can include in the commit log?

I suppose this means such devices are potentially broken for s390 and
jailhouse as well?

> Cc: stable@vger.kernel.org
> Fixes: a02fd05661d73a8 ("PCI: Extend isolated function probing to LoongArch")

a02fd05661d7 (12-char SHA1 is conventional)

> -static inline bool hypervisor_isolated_pci_functions(void)
> +static inline bool hypervisor_isolated_pci_functions(int bus)
>  {
>  	if (IS_ENABLED(CONFIG_S390))
>  		return true;
>  
> -	if (IS_ENABLED(CONFIG_LOONGARCH))
> -		return true;
> +	if (IS_ENABLED(CONFIG_LOONGARCH)) {
> +		if (bus == 0)

I don't really like this embedded assumption that the root bus is bus
00.  That's not necessarily the case; we have many host bridges that
lead to a bus other than 00.

> +			return true;
> +	}
>  
>  	return jailhouse_paravirt();
>  }
> -- 
> 2.47.3
>