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From:   Kai-Heng Feng <kai.heng.feng@canonical.com>
To:     joro@8bytes.org
Cc:     Kai-Heng Feng <kai.heng.feng@canonical.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        iommu@lists.linux-foundation.org (open list:AMD IOMMU (AMD-VI)),
        linux-kernel@vger.kernel.org (open list)
Subject: [PATCH v3] iommu/amd: Disable IOMMU on Stoney Ridge systems
Date:   Mon, 10 Feb 2020 15:51:15 +0800
Message-Id: <20200210075115.14838-1-kai.heng.feng@canonical.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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>
---
v3:
 - Update commit message to mention identity-mapping and ATS don't help.

v2:
 - Find Stoney graphics instead of host bridge.

 drivers/iommu/amd_iommu_init.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/amd_iommu_init.c b/drivers/iommu/amd_iommu_init.c
index 2759a8d57b7f..6be3853a5d97 100644
--- 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;
 		}
-- 
2.17.1