linux-kernel - RE: PCIe host controller behind IOMMU on ARM

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Date:	Wed, 4 Nov 2015 15:19:13 +0000
From:	Phil Edworthy <phil.edworthy@...esas.com>
To:	"Liviu.Dudau@....com" <Liviu.Dudau@....com>
CC:	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Arnd Bergmann <arnd@...db.de>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Magnus <magnus.damm@...il.com>
Subject: RE: PCIe host controller behind IOMMU on ARM

Hi Liviu,

On 04 November 2015 15:02, Liviu wrote:
> On Wed, Nov 04, 2015 at 02:48:38PM +0000, Phil Edworthy wrote:
> > 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 don’t 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.
> 
> Why do you think that? Remember that the only thing attached to the IOMMU is
> the
> host controller. The endpoint is on the PCIe bus, which gets a different
> translation
> that the IOMMU knows nothing about. If it helps you to visualise it better, think
> of the host controller as another IOMMU device. It's the ops of the host
> controller
> that should be invoked, not the IOMMU's.
Ok, that makes sense. I'll have a think and poke it a bit more...

Thanks for your comments
Phil