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SubjectRe: [PATCH v2] iommu/amd: Disable IOMMU on Stoney Ridge systems
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> On Dec 20, 2019, at 10:13, Kai-Heng Feng <kai.heng.feng@canonical.com> wrote:
>
>
>
>> On Dec 20, 2019, at 03:15, Deucher, Alexander <Alexander.Deucher@amd.com> wrote:
>>
>>> -----Original Message-----
>>> From: Kai-Heng Feng <kai.heng.feng@canonical.com>
>>> Sent: Wednesday, December 18, 2019 12:45 PM
>>> To: Joerg Roedel <joro@8bytes.org>
>>> Cc: Christoph Hellwig <hch@infradead.org>; Deucher, Alexander
>>> <Alexander.Deucher@amd.com>; iommu@lists.linux-foundation.org; Kernel
>>> development list <linux-kernel@vger.kernel.org>
>>> Subject: Re: [PATCH v2] iommu/amd: Disable IOMMU on Stoney Ridge
>>> systems
>>>
>>>
>>>
>>>> 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:
>>
>> Does reverting the patch to disable ATS along with this patch help?
>
> Unfortunately it doesn't help.

Any further suggestion to let me try?

Kai-Heng

>
> Kai-Heng
>
>>
>> Alex
>>
>>>
>>> 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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