RE: PCIe host controller behind IOMMU on ARM

From: Phil Edworthy
Date: Wed Nov 04 2015 - 09:48:50 EST

Hi Liviu,

On 04 November 2015 14:24, Liviu wrote:
> On Wed, Nov 04, 2015 at 01:57:48PM +0000, Phil Edworthy wrote:
> > Hi,
> >
> > I am trying to hook up a PCIe host controller that sits behind an IOMMU,
> > but having some problems.
> >
> > I'm using the pcie-rcar PCIe host controller and it works fine without
> > the IOMMU, and I can attach the IOMMU to the controller such that any calls
> > to dma_alloc_coherent made by the controller driver uses the iommu_ops
> > version of dma_ops.
> >
> > However, I can't see how to make the endpoints to utilise the dma_ops that
> > the controller uses. Shouldn't the endpoints inherit the dma_ops from the
> > controller?
> No, not directly.
> > Any pointers for this?
> You need to understand the process through which a driver for endpoint get
> an address to be passed down to the device. Have a look at
> Documentation/DMA-API-HOWTO.txt, there is a nice explanation there.
> (Hint: EP driver needs to call dma_map_single).
> Also, you need to make sure that the bus address that ends up being set into
> the endpoint gets translated correctly by the host controller into an address
> that the IOMMU can then translate into physical address.
Sure, though since this is bog standard Intel PCIe ethernet card which works
fine when the IOMMU is effectively unused, I donât think there is a problem
with that.

The driver for the PCIe controller sets up the IOMMU mapping ok when I
do a test call to dma_alloc_coherent() in the controller's driver. i.e. when I
do this, it ends up in arm_iommu_alloc_attrs(), which calls
__iommu_alloc_buffer() and __alloc_iova().

When an endpoint driver allocates and maps a dma coherent buffer it
also needs to end up in arm_iommu_alloc_attrs(), but it doesn't.