linux-kernel - Re: [PATCH v2] iommu/amd: Disable IOMMU on Stoney Ridge systems

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Date:   Thu, 19 Dec 2019 01:45:12 +0800
From:   Kai-Heng Feng <kai.heng.feng@...onical.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Alex Deucher <alexander.deucher@....com>,
        iommu@...ts.linux-foundation.org,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] iommu/amd: Disable IOMMU on Stoney Ridge systems



> On Dec 17, 2019, at 17:53, Joerg Roedel <joro@...tes.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@...radead.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