Re: PCIe host controller behind IOMMU on ARM

From: Arnd Bergmann
Date: Thu Nov 12 2015 - 04:50:13 EST

On Thursday 12 November 2015 09:26:33 Phil Edworthy wrote:
> On 11 November 2015 18:25, LIviu wrote:
> > On Mon, Nov 09, 2015 at 12:32:13PM +0000, Phil Edworthy wrote:

> > I think you're mixing things a bit or not explaining them very well. Having the
> > PCIe controller limited to 32-bit AXI does not mean that the PCIe bus cannot
> > carry 64-bit addresses. It depends on how they get translated by the host bridge
> > or its associated ATS block. I can't see why you can't have a setup where
> > the CPU addresses are 32-bit but the PCIe bus addresses are all 64-bit.
> > You just have to be careful on how you setup your mem64 ranges so that they
> > don't
> > overlap with the 32-bit ranges when translated.
> From a HW point of view I agree that we can setup the PCI host bridge such that
> it uses 64-bit PCI address, with 32-bit cpu addresses. Though in practice doesn't
> this mean that the dma ops used by card drivers has to be provided by our PCI
> host bridge driver so we can apply the translation to those PCI addresses?
> This comes back to my point below about how to do this. Adding a bus notifier
> to do this may be too late, and arm64 doesn't implement set_dma_ops().
> > And no, you should not limit at the card driver the DMA_BIT_MASK() unless the
> > card is not capable of supporting more than 32-bit addresses.
> If there was infrastructure that checked all parents dma-ranges when the
> dma_set_mask() function is called as Arnd pointed out, this would nicely solve
> the problem.

of_dma_configure calls of_dma_get_range to do all this for the PCIe host,
and then calls arch_setup_dma_ops() so the architecture specific code can
enforce the limits in dma_set_mask and pick an appropriate set of dma
operations. The missing part is in the implementation of arch_setup_dma_ops,
which currently happily ignores the base and limit.

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