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

[Danny Gonzalez <digonzal@google.com>]

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.

Update pci_ats_supported() to walk upstream bridges and return false if
any bridge has PCI_ACS_TB set, 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>
---
 drivers/pci/ats.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
index 9cb23780093d..094f4f825acf 100644
--- a/drivers/pci/ats.c
+++ b/drivers/pci/ats.c
@@ -31,6 +31,24 @@ void pci_ats_init(struct pci_dev *dev)
 	dev->ats_cap = pos;
 }
 
+static struct pci_dev *pci_acs_tb_path_blocked(struct pci_dev *dev)
+{
+	struct pci_dev *bridge;
+
+	for (bridge = pci_upstream_bridge(dev); bridge;
+	     bridge = pci_upstream_bridge(bridge)) {
+		u16 ctrl;
+
+		if (!bridge->acs_cap)
+			continue;
+		pci_read_config_word(bridge, bridge->acs_cap + PCI_ACS_CTRL,
+				     &ctrl);
+		if (ctrl & PCI_ACS_TB)
+			return bridge;
+	}
+	return NULL;
+}
+
 /**
  * pci_ats_supported - check if the device can use ATS
  * @dev: the PCI device
@@ -40,12 +58,22 @@ void pci_ats_init(struct pci_dev *dev)
  */
 bool pci_ats_supported(struct pci_dev *dev)
 {
+	struct pci_dev *bridge;
+
 	if (!dev->ats_cap || dev->untrusted)
 		return false;
 
 	if (dev->is_virtfn)
 		return pci_ats_supported(pci_physfn(dev));
 
+	/* Do not allow ATS if any upstream bridge blocks translation */
+	bridge = pci_acs_tb_path_blocked(dev);
+	if (bridge) {
+		pci_warn_once(dev, "ATS disabled: upstream bridge %s has ACS Translation Blocking enabled\n",
+			      pci_name(bridge));
+		return false;
+	}
+
 	return true;
 }
 EXPORT_SYMBOL_GPL(pci_ats_supported);
-- 
2.56.0.218.g1ec904b173-goog