[PATCH v6 8/8] arm: dma-mapping: plumb our iommu mapping ops into arch_setup_dma_ops

[Thierry Reding]

On Fri, Jan 16, 2015 at 01:18:21AM +0200, Laurent Pinchart wrote:
> On Thursday 15 January 2015 11:12:17 Will Deacon wrote:
> > On Thu, Jan 15, 2015 at 08:28:44AM +0000, Thierry Reding wrote:
> > > On Wed, Jan 14, 2015 at 10:46:10AM +0000, Will Deacon wrote:
> > > > On Wed, Jan 14, 2015 at 09:00:24AM +0000, Alexandre Courbot wrote:
> > > [...]
> > > 
> > >>> 2) Say you want to use the IOMMU API in your driver, and have an iommu
> > >>> property in your device's DT node. If by chance your IOMMU is
> > >>> registered early, you will already have a mapping automatically
> > >>> created even before your probe function is called. Can this be
> > >>> avoided? Is it even safe?
> > >> 
> > >> Currently, I think you have to either teardown the ops manually or
> > >> return an error from of_xlate. Thierry was also looking at this sort of
> > >> thing, so it might be worth talking to him.
> > > 
> > > I already explained in earlier threads why I think this is a bad idea.
> > > It's completely unnatural for any driver to manually tear down something
> > > that it didn't want set up in the first place. It also means that you
> > > have to carefully audit any users of these IOMMU APIs to make sure that
> > > they do tear down. That doesn't sound like a good incremental approach,
> > > as evidenced by the breakage that Alex and Heiko have encountered.
> > 
> > Well, perhaps we hide that behind a get_iommu API or something. We *do*
> > need this manual teardown step to support things like VFIO, so it makes
> > sense to reuse it for other users too imo.
> > 
> > > The solution for me has been to completely side-step the issue and not
> > > register the IOMMU with the new mechanism at all. That is, there's no
> > > .of_xlate() implementation, which means that the ARM DMA API glue won't
> > > try to be smart and use the IOMMU in ways it's not meant to be used.
> 
> That will break when someone will want to use the same IOMMU type for devices 
> that use the DMA mapping API to hide the IOMMU. That might not be the case for 
> your IOMMU today, but it's pretty fragile, we need to fix it.

No, there's absolutely no issue here. It simply means that you can't do
this on Tegra. So far I'm not sure I even see an advantage in using the
IOMMU for devices that don't care about it anyway. Consider the example
of the SD/MMC or HDA. They typically allocate fairly small buffers, the
order of a single page typically. They can simply use memory handed out
by the CMA.

So as long as we don't add a .of_xlate() implementation or instantiate
via the IOMMU_OF_DECLARE() mechanism we simply don't support IOMMU-over-
DMA on Tegra.

Thierry
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