[PATCH] iommu/amd: Disable IOMMU on Stoney Ridge systems

[Kai-Heng Feng]

Serious screen flickering when Stoney Ridge outputs to a 4K monitor.

According to Alex Deucher, IOMMU isn't enabled on Windows, so let's do
the same here to avoid screen flickering on 4K monitor.

Cc: Alex Deucher <alexander.deucher@xxxxxxx>
Bug: https://gitlab.freedesktop.org/drm/amd/issues/961
Signed-off-by: Kai-Heng Feng <kai.heng.feng@xxxxxxxxxxxxx>
---
 drivers/iommu/amd_iommu_init.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/amd_iommu_init.c b/drivers/iommu/amd_iommu_init.c
index 568c52317757..e05f1b269be6 100644
--- a/drivers/iommu/amd_iommu_init.c
+++ b/drivers/iommu/amd_iommu_init.c
@@ -2516,6 +2516,7 @@ static int __init early_amd_iommu_init(void)
 	struct acpi_table_header *ivrs_base;
 	acpi_status status;
 	int i, remap_cache_sz, ret = 0;
+	u32 pci_id;
 
 	if (!amd_iommu_detected)
 		return -ENODEV;
@@ -2603,6 +2604,13 @@ static int __init early_amd_iommu_init(void)
 	if (ret)
 		goto out;
 
+	/* Get the host bridge VID/PID and disable IOMMU if it's Stoney Ridge */
+	pci_id = read_pci_config(0, 0, 0, 0);
+	if ((pci_id & 0xffff) == 0x1022 && (pci_id >> 16) == 0x1576) {
+		pr_info("Disable IOMMU on Stoney Ridge\n");
+		amd_iommu_disabled = true;
+	}
+
 	/* Disable any previously enabled IOMMUs */
 	if (!is_kdump_kernel() || amd_iommu_disabled)
 		disable_iommus();
@@ -2711,7 +2719,7 @@ static int __init state_next(void)
 		ret = early_amd_iommu_init();
 		init_state = ret ? IOMMU_INIT_ERROR : IOMMU_ACPI_FINISHED;
 		if (init_state == IOMMU_ACPI_FINISHED && amd_iommu_disabled) {
-			pr_info("AMD IOMMU disabled on kernel command-line\n");
+			pr_info("AMD IOMMU disabled\n");
 			init_state = IOMMU_CMDLINE_DISABLED;
 			ret = -EINVAL;
 		}
-- 
2.17.1