Re: [PATCH v2] PCI/ATS: Disable ATS when ACS TB is set upstream

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc other folks interested in ATS]

On Wed, Oct 28, 2026 at 11:36:51PM +0000, Danny Gonzalez wrote:
> Enabling ATS allows an endpoint to cache translations and initiate DMA
> requests with Translated addresses, which an upstream bridge with ACS
> Translation Blocking (PCI_ACS_TB) set will block as ACS Violations.
> Because pci_enable_acs() preserves existing PCI_ACS_CTRL bits (e.g., from
> firmware, a kexec kernel, or "pci=config_acs=") without relaxing them,
> PCI_ACS_TB may remain set on upstream bridges.
> 
> Add pci_ats_check_acs() and call it from pci_dma_configure() after
> pci_enable_acs() to walk upstream bridges and clear dev->ats_cap if any
> bridge has PCI_ACS_TB enabled, leaving the endpoint to use Untranslated
> addresses.

> Tested by booting a machine with an endpoint advertising the ATS Capability
> behind a Root Port with PCI_ACS_TB set; verified ATS is disabled and the
> system boots cleanly.
> 
> Signed-off-by: Danny Gonzalez <digonzal@google.com>
> ---
> 
> v2 (based on sashiko review):
>  - Move the upstream ACS TB check from pci_ats_supported() into
>    pci_ats_check_acs() called after pci_enable_acs()
>  - Use pci_acs_enabled() instead of reading PCI_ACS_CTRL directly
> 
>  drivers/pci/ats.c        | 20 ++++++++++++++++++++
>  drivers/pci/pci-driver.c |  1 +
>  drivers/pci/pci.h        |  2 ++
>  3 files changed, 23 insertions(+)
> 
> diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
> index 9cb23780093d..0395612f3987 100644
> --- a/drivers/pci/ats.c
> +++ b/drivers/pci/ats.c
> @@ -31,6 +31,26 @@ void pci_ats_init(struct pci_dev *dev)
>  	dev->ats_cap = pos;
>  }
>  
> +void pci_ats_check_acs(struct pci_dev *dev)
> +{
> +	struct pci_dev *bridge;
> +
> +	if (!dev->ats_cap || dev->is_virtfn || dev->ats_stu)
> +		return;
> +
> +	/* Do not allow ATS if any upstream bridge blocks translation */
> +	for (bridge = pci_upstream_bridge(dev); bridge;
> +	     bridge = pci_upstream_bridge(bridge)) {
> +		if ((bridge->acs_capabilities & PCI_ACS_TB) &&
> +		    pci_acs_enabled(bridge, PCI_ACS_TB)) {
> +			pci_warn_once(dev, "ATS disabled: upstream bridge %s has ACS Translation Blocking enabled\n",
> +				 pci_name(bridge));
> +			dev->ats_cap = 0;
> +			return;
> +		}
> +	}

This works by making pci_ats_supported() return "false".  Can we do
this checking inside pci_ats_supported() instead?  Then we wouldn't
need a new function, and I think it would be easier to understand the
control flow that way.

> +}
> +
>  /**
>   * pci_ats_supported - check if the device can use ATS
>   * @dev: the PCI device
> diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> index e16aa59dd7ac..9a1856def5bd 100644
> --- a/drivers/pci/pci-driver.c
> +++ b/drivers/pci/pci-driver.c
> @@ -1713,6 +1713,7 @@ static int pci_dma_configure(struct device *dev)
>  	 * quirks.
>  	 */
>  	pci_enable_acs(to_pci_dev(dev));
> +	pci_ats_check_acs(to_pci_dev(dev));
>  	pci_put_host_bridge_device(bridge);
>  
> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> index ba3c3fddddc2..cf2c93761d81 100644
> --- a/drivers/pci/pci.h
> +++ b/drivers/pci/pci.h
> @@ -961,9 +961,11 @@ static inline void pcie_walk_rcec(struct pci_dev *rcec,
>  #ifdef CONFIG_PCI_ATS
>  /* Address Translation Service */
>  void pci_ats_init(struct pci_dev *dev);
> +void pci_ats_check_acs(struct pci_dev *dev);
>  void pci_restore_ats_state(struct pci_dev *dev);
>  #else
>  static inline void pci_ats_init(struct pci_dev *d) { }
> +static inline void pci_ats_check_acs(struct pci_dev *d) { }
>  static inline void pci_restore_ats_state(struct pci_dev *dev) { }
>  #endif /* CONFIG_PCI_ATS */
>  
> -- 
> 2.56.0.313.g2479322e68-goog
>