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

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Message-ID: <PS1PR06MB11804263DD55C54C5987654EF52A0@PS1PR06MB1180.apcprd06.prod.outlook.com>
Date:	Wed, 4 Nov 2015 14:48:38 +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 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.

Thanks
Phil