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SubjectRE: [PATCH v2] iommu/amd: Disable IOMMU on Stoney Ridge systems
Date
> -----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?

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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