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

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Date:	Wed, 4 Nov 2015 15:30:29 +0000
From:	Will Deacon <will.deacon@....com>
To:	Phil Edworthy <phil.edworthy@...esas.com>
Cc:	"Liviu.Dudau@....com" <Liviu.Dudau@....com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Magnus <magnus.damm@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: PCIe host controller behind IOMMU on ARM

On Wed, Nov 04, 2015 at 03:19:13PM +0000, Phil Edworthy wrote:
> On 04 November 2015 15:02, Liviu wrote:
> > On Wed, Nov 04, 2015 at 02:48:38PM +0000, Phil Edworthy wrote:
> > > 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...

Take a look at of_iommu_configure, which is currently lacking support
for PCI devices. It should be using a variant on the device-tree bindings
already in use for describing MSI device IDs, so that we can translate
the RequesterID of the endpoint into an ID that the IOMMU can understand.

Will
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