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SubjectRe: [PATCH v2] iommu/amd: Disable IOMMU on Stoney Ridge systems
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> On Dec 17, 2019, at 17:53, Joerg Roedel <joro@8bytes.org> wrote:
>
> On Fri, Dec 06, 2019 at 01:57:41PM +0800, Kai-Heng Feng wrote:
>> Hi Joerg,
>>
>>> On Dec 3, 2019, at 01:00, Christoph Hellwig <hch@infradead.org> wrote:
>>>
>>> On Fri, Nov 29, 2019 at 10:21:54PM +0800, Kai-Heng Feng wrote:
>>>> 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.
>>>
>>> Disabling the IOMMU entirely seem pretty severe. Isn't it enough to
>>> identity map the GPU device?
>>
>> Ok, there's set_device_exclusion_range() to exclude the device from IOMMU.
>> However I don't know how to generate range_start and range_length, which are read from ACPI.
>
> set_device_exclusion_range() is not the solution here. The best is if
> the GPU device is put into a passthrough domain at boot, in which it
> will be identity mapped. DMA still goes through the IOMMU in this case,
> but it only needs to lookup the device-table, page-table walks will not
> be done anymore.
>
> The best way to implement this is to put it into the
> amd_iommu_add_device() in drivers/iommu/amd_iommu.c. There is this
> check:
>
> if (dev_data->iommu_v2)
> iommu_request_dm_for_dev(dev);
>
> The iommu_request_dm_for_dev() function causes the device to be identity
> mapped. The check can be extended to also check for a device white-list
> for devices that need identity mapping.

My patch looks like this but the original behavior (4K screen flickering) is still the same:

diff --git a/drivers/iommu/amd_iommu.c b/drivers/iommu/amd_iommu.c
index bd25674ee4db..f913a25c9e92 100644
--- a/drivers/iommu/amd_iommu.c
+++ b/drivers/iommu/amd_iommu.c
@@ -42,6 +42,7 @@
#include <asm/iommu.h>
#include <asm/gart.h>
#include <asm/dma.h>
+#include <asm/pci-direct.h>

#include "amd_iommu_proto.h"
#include "amd_iommu_types.h"
@@ -2159,6 +2160,8 @@ static int amd_iommu_add_device(struct device *dev)
struct iommu_domain *domain;
struct amd_iommu *iommu;
int ret, devid;
+ bool need_identity_mapping = false;
+ u32 header;

if (!check_device(dev) || get_dev_data(dev))
return 0;
@@ -2184,7 +2187,11 @@ static int amd_iommu_add_device(struct device *dev)

BUG_ON(!dev_data);

- if (dev_data->iommu_v2)
+ header = read_pci_config(0, PCI_BUS_NUM(devid), PCI_SLOT(devid), PCI_FUNC(devid));
+ if ((header & 0xffff) == 0x1002 && (header >> 16) == 0x98e4)
+ need_identity_mapping = true;
+
+ if (dev_data->iommu_v2 || need_identity_mapping)
iommu_request_dm_for_dev(dev);

/* Domains are initialized for this device - have a look what we ended up with */

$ dmesg | grep -i direct
[ 0.011446] Using GB pages for direct mapping
[ 0.703369] pci 0000:00:01.0: Using iommu direct mapping
[ 0.703830] pci 0000:00:08.0: Using iommu direct mapping
So the graphics device (pci 0000:00:01.0:) is using direct mapping after the change.
Kai-Heng

>
> HTH,
>
> Joerg

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