iommu/amd: Disable IOMMU on Stoney Ridge systems

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

Use identity-mapping and PCI ATS doesn't help this issue.

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@amd.com>
Bug: https://gitlab.freedesktop.org/drm/amd/issues/961
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

diff --git a/drivers/iommu/amd_iommu_init.c b/drivers/iommu/amd_iommu_init.c
index 2759a8d57b7f91be441012c23c01bb6d6b408907..6be3853a5d978e09b8aeebb02daaec01e3a80cc9 100644 (file)
--- a/drivers/iommu/amd_iommu_init.c
+++ b/drivers/iommu/amd_iommu_init.c
@@ -2523,6 +2523,7 @@ 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;
@@ -2610,6 +2611,16 @@ static int __init early_amd_iommu_init(void)
        if (ret)
                goto out;
 
+       /* Disable IOMMU if there's Stoney Ridge graphics */
+       for (i = 0; i < 32; i++) {
+               pci_id = read_pci_config(0, i, 0, 0);
+               if ((pci_id & 0xffff) == 0x1002 && (pci_id >> 16) == 0x98e4) {
+                       pr_info("Disable IOMMU on Stoney Ridge\n");
+                       amd_iommu_disabled = true;
+                       break;
+               }
+       }
+
        /* Disable any previously enabled IOMMUs */
        if (!is_kdump_kernel() || amd_iommu_disabled)
                disable_iommus();
@@ -2718,7 +2729,7 @@ 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;
                }