Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

[Dave Martin]

On Fri, May 30, 2014 at 08:30:08AM +0100, Thierry Reding wrote:
> On Thu, May 29, 2014 at 09:52:22AM -0600, Stephen Warren wrote:
> > On 05/23/2014 02:36 PM, Thierry Reding wrote:
> > > From: Thierry Reding <treding@xxxxxxxxxx>
> > > 
> > > This commit introduces a generic device tree binding for IOMMU devices.
> > > Only a very minimal subset is described here, but it is enough to cover
> > > the requirements of both the Exynos System MMU and Tegra SMMU as
> > > discussed here:
> > > 
> > >     https://lkml.org/lkml/2014/4/27/346
> > > 
> > > Signed-off-by: Thierry Reding <treding@xxxxxxxxxx>
> > > ---
> > > Apologies for the noise, but apparently I mistyped one of the email
> > > addresses, should be fixed now.
> > > 
> > > Changes in v2:
> > > - add notes about "dma-ranges" property (drop note from commit message)
> > > - document priorities of "iommus" property vs. "dma-ranges" property
> > > - drop #iommu-cells in favour of #address-cells and #size-cells
> > 
> > I think this is a mistake. address-cells/size-cells are for transactions
> > flowing down the bus (from the CPU to date). Describing a connection
> > from a device to an IOMMU is something completely different, and should
> > therefore simply use an iommu-cells property to describe any necessary
> > information. If we start re-using properties for different things in
> > different contexts, how is anyone going to know what they mean, and how
> > will conflicts be resolved. For example, what if there's a single HW
> > module that both acts as a regular register bus with children (where
> > address-cells/size-cells defines how transactions reach the children
> > from the parent), and is also an IOMMU (where according to this binding
> > proposal, address-cells/size-cells represent some aspect of the IOMMU
> > feature). Using different properties for different things is the only
> > sane way to keep different concepts separate. Another alternative would
> > be to represent the single HW module as separate nodes in DT, but I
> > think that will only make our lives harder, and where I've done that in
> > the past, I've regretted it.
> 
> There was some back-and-forth on this topic and the latest concensus
> when I wrote the second version was that #address-cells and #size-cells
> were to be used.
> 
> But there was some bore back-and-forth after that, and it seems like
> Arnd no longer thinks that using #address-cells and #size-cells is a
> good idea either[0].

Mistake or not, ePAPR already (ab)uses the address concept for PCI.
Unless ePAPR is wrong, I don't think it makes sense to argue that
the address concept cannot be repurposed.

The reason why this is abused for PCI is the same as our reason here:
different masters really are treated as distinct even when accessing
the same destination address, and DT has no general native way to
describe that.

One clear advantage of using #address-cells etc. is that ePAPR already
has very clear and well-defined ways of how to specify range mappings.
This gives us a ready-made way to describe windowed IOMMUs and 1:1
remappings of whole blocks of master IDs, which are the most obvious
cases other than having a small-integer set of explicit IDs.

That said, the PCI pseudo-address thing is not something we _necessarily_
want to repeat.

> Arnd, can you take another look at this binding and see if there's
> anything else missing? If not I'll go through the document again and
> update all #address-cells/#size-cells references with #iommu-cells as
> appropriate and submit v3.

How do you envisage propagation of the master ID bits downstream of the
IOMMU would be described?

We will definitely need a way to describe this for GICv3.  How those
values are propagated is likely to vary between related SoCs and doesn't
feel like it should be baked into a driver, especially for the ARM SMMU
which may get reused in radically different SoC families from different
vendors.

The most likely types of remapping are the adding of a base offset or
some extra bits to the ID -- because not all MSIs to the GIC will
necessarily pass through the IOMMU.  It's also possible that we might
see ID squashing or folding in some systems.


For types of remapping which mix the ID and address together, I now
do tend to agree that any flexibility arising from describing that
in a general way that is unlikely to repay the cost of trying to
interpret and analyse the DT.  Defining a few sterotypical kinds
of mapping explicitly, as needed, looks more sensible for now.  The
windowed-IOMMU case is one example.

Cheers
---Dave
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