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    SubjectRe: Question about iommu on x86_64 and radeon driver.
    On Fri, May 27, 2011 at 11:20:09PM +0100, Daniel Haid wrote:
    > >The DRM code is not used anymore. It was used by XFree86 but nowadays
    > >the TTM code is used instead.
    > I see
    > >Ah, this would be a lengthy writeup.. so did you look at the
    > >link I pointed you to?
    > Yes I have started, but do not understand everything yet.
    > What are MFNs? Is this something Xen-specific?

    > An does your post imply that on bare metal with "swiotlb=force
    > iommu=soft"
    > alloc_page with GFP_DMA32 does also not allocate under 4GB?

    Noo.. It does, but the normal assumption of 'phys_to_virt' == 'phys_to_bus' is
    not valid anymore. Think of a buffer (swiotlb) which has a pool
    of pages and when a PCI device wants a page, it hands one out. It also has
    other functionality such as 'mapping' of an already allocated page. If the
    PCI device asks the IOMMU (swiotlb) to map a page (and if you have 'swiotlb=force'
    the page provided has been allocated above 4GB and the device can only handle
    up to 32-bit), then swiotlb gives out a page from its own pool. You now have
    two addresses: the one from the PCI pool (swiotlb) and the one you already
    allocated. You are suppose to program your PCI card to read/write data to the
    page provided from the IOMMU (so the swiotlb), which means that it won't
    write to the page you had allocated. Hence there are a calls, such as 'sync_page'..
    which will copy the contents from the swiotlb page to the one you had allocated
    (or vice-versa). This is called 'bounce buffer'.

    The radeon (and nouveau) don't have the code to call the 'sync_page', and
    you wouldn't really want to do so - as it slows down the performance of the
    machine. There exists another mechanism which is to allocate the pages
    at the start, and not do mapping later one.

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