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    SubjectRe: [PATCH] AMD IOMMU: revert "x86, AMD IOMMU: honor iommu=off instead of amd_iommu=off"
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    On Mon, 22 Sep 2008 16:27:13 +0200
    Joerg Roedel <joerg.roedel@amd.com> wrote:

    > On Mon, Sep 22, 2008 at 11:16:25PM +0900, FUJITA Tomonori wrote:
    > > On Mon, 22 Sep 2008 16:07:49 +0200
    > > Joerg Roedel <joerg.roedel@amd.com> wrote:
    > >
    > > > On Mon, Sep 22, 2008 at 10:35:08PM +0900, FUJITA Tomonori wrote:
    > > > > This is against tip/x86/iommu.
    > > > >
    > > > > =
    > > > > From: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
    > > > > Subject: [PATCH] AMD IOMMU: revert "x86, AMD IOMMU: honor iommu=off instead of amd_iommu=off"
    > > > >
    > > > > This reverts:
    > > > >
    > > > > commit 8b14518fadd9d5915827d86d5c10e602fedf042e
    > > > > Author: Joerg Roedel <joerg.roedel@amd.com>
    > > > > Date: Thu Jul 3 19:35:09 2008 +0200
    > > > >
    > > > > x86, AMD IOMMU: honor iommu=off instead of amd_iommu=off
    > > > >
    > > > > This patch removes the amd_iommu=off kernel parameter and honors the generic
    > > > >
    > > > > iommu=off parameter for the same purpose.
    > > > >
    > > > >
    > > > > The above commit is wrong.
    > > >
    > > > It isn't. The user normally don't care about the type of IOMMU in the
    > > > system. So disabling it with iommu=off is the right way. To achieve what
    > > > you want its better to add iommu=gart and iommu=amd to the option
    > > > parser. This will be consistent with Calgary and SWIOTLB too.
    > >
    > > I'm not talking about what the best way to disable IOMMUs for
    > > users. I'm talking about how the current code works. It's a different
    > > topic. If you want to fix this, that's fine.
    > >
    > > I just pointed out:
    > >
    > > - You think that iommu=off and amd_iommu=off worked in the same way,
    > > but they didn't.
    > >
    > > - This commit removed the useful feature.
    >
    > Ok, let me explain it this way. Before there was an AMD IOMMU driver
    > the user had to pass iommu=off to disable iommu usage at all and
    > iommu=soft to just disable usage of hardware IOMMUs.

    iommu=soft is not the correct way to disable usage of hardware
    IOMMUs. You don't always need SWIOTLB. pci-nommu.c is better if
    possible.

    > Further you have iommu=calgary to select the Calgary driver.

    I don't think that users need to have iommu=calgary parameter to use
    calgary IOMMU by default. If you enable CONFIG_CALGARY_IOMMU,
    CALGARY_IOMMU_ENABLED_BY_DEFAULT is also enabled by default. If a
    kernel finds calgary IOMMU, the kernel uses it by default.

    It's also consistent with how SWIOTLB. Users don't need iommu=soft
    parameter to enable SWIOTLB. A kernel enables SWIOTLB automatically
    when necessary.


    Well, this is the problem about the IOMMUs parameters. The IOMMUs
    parameters are too complicated for everyone.


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