linux-kernel - Re: [PATCH v2] PCI: Increase maximum PCIe physical function number to 7 for non-ARI devices

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Date: Fri, 16 Feb 2024 14:26:47 -0800
From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
To: Bean Huo <beanhuo@...pp.de>, bhelgaas@...gle.com, schnelle@...ux.ibm.com
Cc: linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
 Bean Huo <beanhuo@...ron.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2] PCI: Increase maximum PCIe physical function number to
 7 for non-ARI devices


On 2/16/24 11:01 AM, Bean Huo wrote:
> From: Bean Huo <beanhuo@...ron.com>
>
> The PCIe specification allows up to 8 Physical Functions (PFs) per endpoint
> when ARI (Alternative Routing-ID Interpretation) is not supported. Previously,
> our implementation erroneously limited the maximum number of PFs to 7 for
> endpoints without ARI support.
I would quote specification reference here, like below:

As per PCIe specification r6.2, sec titled "Alternative Routing-ID Interpretation
(ARI)", up to 8 [fn # 0..7] functions are allowed in an non ARI capable device.

It looks like for an ARI capable device the limit is 256. Why not add that
check as well?

>
> This patch corrects the maximum PF count to adhere to the PCIe specification
> by allowing up to 8 PFs on non-ARI endpoints. This change ensures better
> compliance with the standard and improves compatibility with devices relying
> on this specification.
>
> The necessity for this adjustment was verified by a thorough review of the
> "Alternative Routing-ID Interpretation (ARI)" section in the PCIe 3.0 Spec,
> which first introduced ARI.

I would always use latest PCIe spec for reference.

>
> Fixes: c3df83e01a96 ("PCI: Clean up pci_scan_slot()")
> Cc: stable@...r.kernel.org
> Signed-off-by: Bean Huo <beanhuo@...ron.com>
> ---
> Changelog:
> 	v1--v2:
> 		1. Add Fixes tag
> 		2. Modify commit message
> ---
>  drivers/pci/probe.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index ed6b7f48736a..8c3d0f63bc13 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2630,7 +2630,8 @@ static int next_fn(struct pci_bus *bus, struct pci_dev *dev, int fn)
>  	if (pci_ari_enabled(bus))
>  		return next_ari_fn(bus, dev, fn);
>  
> -	if (fn >= 7)
> +	/* If EP does not support ARI, the maximum number of functions should be 7 */
> +	if (fn > 7)
>  		return -ENODEV;
>  	/* only multifunction devices may have more functions */
>  	if (dev && !dev->multifunction)

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer