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From: Joerg Roedel <joro@8bytes.org>
To: Kai-Heng Feng <kai.heng.feng@canonical.com>
Cc: Alex Deucher <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
Date: Tue, 17 Dec 2019 10:53:41 +0100	[thread overview]
Message-ID: <20191217095341.GG8689@8bytes.org> (raw)
In-Reply-To: <974A8EB3-70B6-4A33-B36C-CFF69464493C@canonical.com>

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.

HTH,

	Joerg

_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu

WARNING: multiple messages have this Message-ID (diff)
From: Joerg Roedel <joro@8bytes.org>
To: Kai-Heng Feng <kai.heng.feng@canonical.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	Alex Deucher <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
Date: Tue, 17 Dec 2019 10:53:41 +0100	[thread overview]
Message-ID: <20191217095341.GG8689@8bytes.org> (raw)
In-Reply-To: <974A8EB3-70B6-4A33-B36C-CFF69464493C@canonical.com>

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.

HTH,

	Joerg


  reply	other threads:[~2019-12-17  9:53 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-29 14:21 [PATCH v2] iommu/amd: Disable IOMMU on Stoney Ridge systems Kai-Heng Feng
2019-11-29 14:21 ` Kai-Heng Feng
2019-12-01 12:42 ` Lucas Stach
2019-12-01 12:42   ` Lucas Stach
2019-12-02 16:36   ` Deucher, Alexander
2019-12-02 16:36     ` Deucher, Alexander
2019-12-04 16:08     ` Deucher, Alexander
2019-12-04 16:08       ` Deucher, Alexander
2019-12-06  5:59       ` Kai-Heng Feng
2019-12-06  5:59         ` Kai-Heng Feng
2019-12-06  8:57       ` Christian König
2019-12-06  8:57         ` Christian König
2019-12-02 17:00 ` Christoph Hellwig
2019-12-02 17:00   ` Christoph Hellwig
2019-12-06  5:57   ` Kai-Heng Feng
2019-12-06  5:57     ` Kai-Heng Feng
2019-12-17  9:53     ` Joerg Roedel [this message]
2019-12-17  9:53       ` Joerg Roedel
2019-12-18 17:45       ` Kai-Heng Feng
2019-12-18 17:45         ` Kai-Heng Feng
2019-12-19 19:15         ` Deucher, Alexander
2019-12-19 19:15           ` Deucher, Alexander
2019-12-20  2:13           ` Kai-Heng Feng
2019-12-20  2:13             ` Kai-Heng Feng
2020-01-06  8:37             ` Kai-Heng Feng
2020-01-06  8:37               ` Kai-Heng Feng
2020-02-05 10:10               ` Kai-Heng Feng
2020-02-05 10:10                 ` Kai-Heng Feng
2020-02-05 13:24                 ` Joerg Roedel
2020-02-05 13:24                   ` Joerg Roedel

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