Messages in this thread Patch in this message | | | Subject | Re: [PATCH v2] iommu/amd: Disable IOMMU on Stoney Ridge systems | From | Kai-Heng Feng <> | Date | Thu, 19 Dec 2019 01:45:12 +0800 |
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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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