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

Arnd Bergmann
2014-05-20T20:26:12Z

On Tuesday 20 May 2014 16:24:59 Dave Martin wrote:
> On Tue, May 20, 2014 at 02:41:18PM +0200, Arnd Bergmann wrote:
> > On Tuesday 20 May 2014 14:02:43 Thierry Reding wrote:
> > > On Tue, May 20, 2014 at 01:15:48PM +0200, Arnd Bergmann wrote:
> > > Typical values for the above include:
> > > - #address-cells = <0>, size-cells = <0>: Single master IOMMU devices are not
> > > configurable and therefore no additional information needs to be encoded in
> > > the specifier. This may also apply to multiple master IOMMU devices that do
> > > not allow the association of masters to be configured.
> > > - #address-cells = <1>, size-cells = <0>: Multiple master IOMMU devices may
> > > need to be configured in order to enable translation for a given master. In
> > > such cases the single address cell corresponds to the master device's ID.
> > > - #address-cells = <2>, size-cells = <2>: Some IOMMU devices allow the DMA
> > > window for masters to be configured. The first cell of the address in this
> > > may contain the master device's ID for example, while the second cell could
> > > contain the start of the DMA window for the given device. The length of the
> > > DMA window is specified by two additional cells.
>
> I was trying to figure out how to describe the different kinds of
> transformation we could have on the address/ID input to the IOMMU.
> Treating the whole thing as opaque gets us off the hook there.
>
> IDs are probably not propagated, not remapped, or we simply don't care
> about them; whereas the address transformation is software-controlled,
> so we don't describe that anyway.
>
> Delegating grokking the mapping to the iommu driver makes sense --
> it's what it's there for, after all.
>
>
> I'm not sure whether the windowed IOMMU case is special actually.
>
> Since the address to program into the master is found by calling the
> IOMMU driver to create some mappings, does anything except the IOMMU
> driver need to understand that there is windowing?

No. I tried to explain that earlier today, and in my earlier mails
I hadn't thought that part through. Only the IOMMU driver needs to care
about the window.

> > >
> > > Examples:
> > > =========
> > >
> > > Single-master IOMMU:
> > > --------------------
> > >
> > > iommu {
> > > #address-cells = <0>;
> > > #size-cells = <0>;
> > > };
> > >
> > > master {
> > > iommus = <&/iommu>;
> > > };
> > >
> > > Multiple-master IOMMU with fixed associations:
> > > ----------------------------------------------
> > >
> > > /* multiple-master IOMMU */
> > > iommu {
> > > /*
> > > * Masters are statically associated with this IOMMU and
> > > * address translation is always enabled.
> > > */
> > > #iommu-cells = <0>;
> > > };
> >
> > copied wrong? I guess you mean #address-cells=<0>/#size-cells=<0> here.
> >
> > > /* static association with IOMMU */
> > > master@1 {
> > > reg = <1>;
>
> Just for clarification, "reg" just has its standard meaning here, and
> is nothing to do with the IOMMU?

correct

> > > iommus = <&/iommu>;
>
> In effect, "iommus" is doing the same thing as my "slaves" property.
>
> The way #address-cells and #size-cells determine the address and range
> size for mastering into the IOMMU is also similar. The main difference
> is that I didn't build the ID into the address.

Right. I think the difference is more about what we want to call
things: Calling it iommu means we want to specifically describe
the case of iommus that needs to get handled by all OSs in a particular
way, while the more generic slave connection doesn't correspond to
a specific concept in the OS.

> > > };
> > >
> > > /* static association with IOMMU */
> > > master@2 {
> > > reg = <2>;
> > > iommus = <&/iommu>;
> > > };
> > >
> > > Multiple-master IOMMU:
> > > ----------------------
> > >
> > > iommu {
> > > /* the specifier represents the ID of the master */
> > > #address-cells = <1>;
> > > #size-cells = <0>;
>
> How do we know the size of the input address to the IOMMU? Do we
> get cases for example where the IOMMU only accepts a 32-bit input
> address, but some 64-bit capable masters are connected through it?

I was stuck on this question for a while before, but then I realized
that it doesn't matter at all: It's the IOMMU driver itself that
manages the address space, and it doesn't matter if a slave can
address a larger range than the IOMMU can accept. If the IOMMU
needs to deal with the opposite case (64-bit input addresses
but a 32-bit master), that limitation can be put into the specifier.

> The size of the output address from the IOMMU will be determined
> by its own mastering destination, which by default in ePAPR is the
> IOMMU node's parent. I think that's what you intended, and what we
> expect in this case.

Rihgt.

> For determining dma masks, it is the output address that it
> important. Santosh's code can probably be taught to handle this,
> if given an additional traversal rule for following "iommus"
> properties. However, deploying an IOMMU whose output address size
> is smaller than the

Something seems to be missing here. I don't think we want to handle
the case where the IOMMU output cannot the entire memory address
space. If necessary, that would mean using both an IOMMU driver
and swiotlb, but I think it's a reasonable assumption that hardware
isn't /that/ crazy.

> > > Multiple-master device:
> > > -----------------------
> > >
> > > /* single-master IOMMU */
> > > iommu@1 {
> > > reg = <1>;
> > > #address-cells = <0>;
> > > #size-cells = <0>;
> > > };
> > >
> > > /* multiple-master IOMMU */
> > > iommu@2 {
> > > reg = <2>;
> > > #address-cells = <1>;
> > > #size-cells = <0>;
> > > };
> > >
> > > /* device with two master interfaces */
> > > master {
> > > iommus = <&/iommu@1>, /* master of the single-master IOMMU */
> > > <&/iommu@2 42>; /* ID 42 in multiple-master IOMMU */
> > > };
> > >
> > > Multiple-master IOMMU with configurable DMA window:
> > > ---------------------------------------------------
> > >
> > > / {
> > > #address-cells = <1>;
> > > #size-cells = <1>;
> > >
> > > iommu {
> > > /* master ID, address of DMA window */
> > > #address-cells = <2>;
> > > #size-cells = <2>;
> > > };
> > >
> > > master {
> > > /* master ID 42, 4 GiB DMA window starting at 0 */
> > > iommus = <&/iommu 42 0 0x1 0x0>;
> > > };
> > > };
> > >
> > > Does that sound about right?
> >
> > Yes, sounds great. I would probably leave out the Multiple-master device
> > from the examples, since that seems to be a rather obscure case.
>
> I think multi-master is the common case.

Which of the two cases above do you mean? I was referring to the first
as being obscure, not the second.

I still haven't seen an example of the first, while the second one
is very common.

> > ----------->8
> >
> > Does that make sense to you? We can change what we say about
> > dma-ranges, I mainly want to be clear with what is or is not
> > allowed at this point.
>
> I think it would be inconsistent and unnecessary to disallow it in the
> binding. The meaning you've proposed seems completely consistent with
> ePAPR, so I suggest to keep it. The IOMMU is just another bus master
> from the ePAPR point of view -- no need to make special rules for it
> unless they are useful.
>
> The binding does not need to be (and generally shouldn't be) a
> description of precisely what the kernel does and does not support.
>
> However, if we don't need to support non-identity dma-ranges in Linux
> yet, we have the option to barf if we see such a dma-ranges memorywards
> of an IOMMU, if it simplifies the Linux implementation. We could always
> relax that later -- and it'll be obvious how to describe that situation
> in DT.

Ok.

> What I would like to see is a recommandation, based on Thierry's binding
> here, for describing how cross-mastering in general is described. It's
> not really a binding, but more of a template for bindings.
>
> I'm happy to have a go at writing it, then we can decide whether it's
> useful or not.

I don't mind if you take this on, but I'm not sure if that should be
part of this binding or not. Let's see what you come up with.

> There are a few things from the discussion that are *not* solved by this
> iommu binding, but they seem reasonable. The binding also doesn't block
> solving those things later if/when needed:
>
> 1) Cross-mastering to things that are not IOMMUs
>
> We might need to solve this later if we encounter SoCs with
> problematic topologies, we shouldn't worry about it for the time
> being.
>
> We'll to revisit it for GICv3 but that's a separate topic.

> 2) Describing address and ID remappings for cross-mastering.
>
> We can describe this in a way that is consistent with this IOMMU
> binding. We will need to describe something for GICv3, but the
> common case will be that IDs are just passed through without
> remapping.
>
> We don't need to clarify how IDs are propagated until we have
> something in DT for IDs to propagate to.

Ok, thanks for pointing these out. I had forgotten about the MSI
case, but it seems ok to defer that part for now.

Arnd