Re: [PATCH v4 2/3] PCI: Allow ATS to be always on for pre-CXL devices

[Bjorn Helgaas <helgaas@kernel.org>]

On Sun, Apr 26, 2026 at 10:54:01PM -0700, Nicolin Chen wrote:
> Some NVIDIA GPU/NIC devices, though they don't implement CXL config space,
> have many CXL-like properties. Call this kind "pre-CXL".
> 
> Similar to CXL.cache capability, these pre-CXL devices also require the ATS
> function even when their RIDs are IOMMU bypassed, i.e. keep ATS "always on"
> v.s. "on demand" when a non-zero PASID line gets enabled in SVA use cases.
> ...

> +/* Some pre-CXL devices require ATS when it is IOMMU-bypassed */

I guess these devices are purely PCIe, with no actual CXL
transactions, so a hint here about what leads to the ATS requirement
would be useful.  It sounds like an actual functional requirement, not
just a performance optimization.

> +bool pci_dev_specific_ats_always_on(struct pci_dev *pdev)
> +{
> +	const struct pci_dev_ats_always_on *i;
> +
> +	for (i = pci_dev_ats_always_on; i->vendor; i++) {
> +		if (i->vendor != pdev->vendor)
> +			continue;
> +		if (i->ats_always_on && i->ats_always_on(pdev))
> +			return true;
> +		if (!i->ats_always_on && i->device == pdev->device)
> +			return true;
> +	}
> +
> +	return false;
> +}
>  #endif /* CONFIG_PCI_ATS */
>  
>  /* Freescale PCIe doesn't support MSI in RC mode */
> -- 
> 2.43.0
>