[PATCH v2] 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.

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;
+		}
+	}
+}
+
 /**
  * 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