Re: [PATCH v4] devicetree: Add generic IOMMU device tree bindings

Mark Rutland
2014-07-31T10:50:17Z

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.