RE: [PATCH v2] iommu/amd: Disable IOMMU on Stoney Ridge systems

["Deucher, Alexander" <Alexander.Deucher@amd.com>]

> -----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