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    SubjectRe: [PATCH v4] devicetree: Add generic IOMMU device tree bindings
    On Thu, Jul 31, 2014 at 11:09:06AM +0100, Thierry Reding wrote:
    > On Wed, Jul 30, 2014 at 07:18:42PM +0100, Mark Rutland wrote:
    > [...]
    > > > >> +
    > > > >> +Multiple-master IOMMU with configurable DMA window:
    > > > >> +---------------------------------------------------
    > > > >> +
    > > > >> + / {
    > > > >> + #address-cells = <1>;
    > > > >> + #size-cells = <1>;
    > > > >> +
    > > > >> + iommu {
    > > > >> + /* master ID, address and length of DMA window */
    > > > >> + #iommu-cells = <4>;
    > > > >> + };
    > > > >> +
    > > > >> + master {
    > > > >> + /* master ID 42, 4 GiB DMA window starting at 0 */
    > > > >> + iommus = <&/iommu 42 0 0x1 0x0>;
    > > > >
    > > > > Is this that window is from the POV of the master, i.e. the master can
    > > > > address 0x0 to 0xffffffff when generating transactions, and these get
    > > > > translated somehow?
    > > > >
    > > > > Or is this the physical addresses to allocate to the master?
    > > >
    > > > It needs to be clarified in the documentation, but as far as I know it
    > > > is the DMA address space that is used.
    > >
    > > Ok. So that's pre-translation, from the POV of the master?
    >
    > Correct. It represents the window of the IOMMU's addressable I/O virtual
    > address space that should be assigned to this particular master.
    >
    > > If we don't have that knowledge about the master already (e.g. based on
    > > the compatible string), surely we always need that information in a
    > > given iommu-specifier format? Otherwise certain iommus won't be able to
    > > handle masters with limited addressing only due to limitations of their
    > > binding.
    >
    > This is only used for what's often called a windowed IOMMU. Many IOMMUs
    > (non-windowed) typically allow only a complete address space to be
    > assigned to a master without additional control over subregions. So this
    > is really a property/capability of the IOMMU rather than the masters
    > themselves.

    I'm not sure I follow, but I'm happy to wait until we have the first
    windowed IOMMU using this binding. I'll try to get myself up to speed in
    the mean time.

    > There are already other means to respect the addressing limitations of
    > masters. We typcially use a device's DMA mask for this, and it's natural
    > to reuse that for I/O virtual addresses since they will in fact take the
    > place of physical addresses for the master when translation is enabled.

    Ok, that covers my worry then.

    Cheers,
    Mark.


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