linux-kernel - Re: [PATCH] PCI/ATS: Allow to enable ATS on VFs even if it is not enabled on PF

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Date:   Sun, 12 Feb 2023 20:14:48 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        bhelgaas@...gle.com, jean-philippe@...aro.org,
        darren@...amperecomputing.com, scott@...amperecomputing.com
Subject: Re: [PATCH] PCI/ATS:  Allow to enable ATS on VFs even if it is not
 enabled on PF

On Wed, Feb 08, 2023 at 10:43:21AM -0800, Ganapatrao Kulkarni wrote:
> As per PCIe specification(section 10.5), If a VF implements an
> ATS capability, its associated PF must implement an ATS capability.
> The ATS Capabilities in VFs and their associated PFs are permitted to
> be enabled independently.
> Also, it states that the Smallest Translation Unit (STU) for VFs must be
> hardwired to Zero and the associated PF's value applies to VFs STU.
> 
> The current code allows to enable ATS on VFs only if it is already
> enabled on associated PF, which is not necessary as per the specification.
> 
> It is only required to have valid STU programmed on PF to enable
> ATS on VFs. Adding code to write the first VFs STU to a PF's STU
> when PFs ATS is not enabled.

Can you please add here quotes from the spec and its version? I don't see
anything like this in my version of PCIe specification.

Thanks

> 
> Signed-off-by: Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
> ---
>  drivers/pci/ats.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
> index f9cc2e10b676..a97ec67201d1 100644
> --- a/drivers/pci/ats.c
> +++ b/drivers/pci/ats.c
> @@ -67,13 +67,20 @@ int pci_enable_ats(struct pci_dev *dev, int ps)
>  	if (ps < PCI_ATS_MIN_STU)
>  		return -EINVAL;
>  
> -	/*
> -	 * Note that enabling ATS on a VF fails unless it's already enabled
> -	 * with the same STU on the PF.
> -	 */
>  	ctrl = PCI_ATS_CTRL_ENABLE;
>  	if (dev->is_virtfn) {
>  		pdev = pci_physfn(dev);
> +
> +		if (!pdev->ats_enabled &&
> +				(pdev->ats_stu < PCI_ATS_MIN_STU)) {
> +			u16 ctrl2;
> +
> +			/* Associated PF's STU value applies to VFs. */
> +			pdev->ats_stu = ps;
> +			ctrl2 = PCI_ATS_CTRL_STU(pdev->ats_stu - PCI_ATS_MIN_STU);
> +			pci_write_config_word(pdev, pdev->ats_cap + PCI_ATS_CTRL, ctrl2);
> +		}
> +
>  		if (pdev->ats_stu != ps)
>  			return -EINVAL;
>  	} else {
> -- 
> 2.39.1
>