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:
[...]

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+
+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.

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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.