[PATCH 1/1] iommu/vt-d: Prevent boot failure with devices requiring ATS

[Lu Baolu <baolu.lu@linux.intel.com>]

SOC-integrated devices on some platforms require their PCI ATS enabled
for operation when the IOMMU is in scalable mode. Those devices are
reported via ACPI/SATC table with the ATC_REQUIRED bit set in the Flags
field.

The PCI subsystem offers the 'pci=noats' kernel command to disable PCI
ATS on all devices. Using 'pci=noat' with devices that require PCI ATS
can cause a conflict, leading to boot failure, especially if the device
is a graphics device.

To prevent this issue, check PCI ATS support before enumerating the IOMMU
devices. If any device requires PCI ATS, but PCI ATS is disabled by
'pci=noats', switch the IOMMU to operate in legacy mode to ensure
successful booting.

Fixes: 97f2f2c5317f ("iommu/vt-d: Enable ATS for the devices in SATC table")
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/12036
Cc: stable@vger.kernel.org
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
---
 drivers/iommu/intel/iommu.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index 4aa070cf56e7..8f275e046e91 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -3127,10 +3127,26 @@ int dmar_iommu_notify_scope_dev(struct dmar_pci_notify_info *info)
 					(void *)satc + satc->header.length,
 					satc->segment, satcu->devices,
 					satcu->devices_cnt);
-			if (ret > 0)
-				break;
-			else if (ret < 0)
+			if (ret < 0)
 				return ret;
+
+			if (ret > 0) {
+				/*
+				 * The device requires PCI/ATS when the IOMMU
+				 * works in the scalable mode. If PCI/ATS is
+				 * disabled using the pci=noats kernel parameter,
+				 * the IOMMU will default to legacy mode. Users
+				 * are informed of this change.
+				 */
+				if (intel_iommu_sm && satcu->atc_required &&
+				    !pci_ats_supported(info->dev)) {
+					pci_warn(info->dev,
+						 "PCI/ATS not supported, system working in IOMMU legacy mode\n");
+					intel_iommu_sm = 0;
+				}
+
+				break;
+			}
 		} else if (info->event == BUS_NOTIFY_REMOVED_DEVICE) {
 			if (dmar_remove_dev_scope(info, satc->segment,
 					satcu->devices, satcu->devices_cnt))
-- 
2.34.1