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

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Date:	Wed, 11 Nov 2015 21:22:35 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	"Liviu.Dudau@....com" <Liviu.Dudau@....com>
Cc:	Phil Edworthy <phil.edworthy@...esas.com>,
	Will Deacon <will.deacon@....com>,
	"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>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Magnus <magnus.damm@...il.com>
Subject: Re: PCIe host controller behind IOMMU on ARM

On Wednesday 11 November 2015 18:24:56 Liviu.Dudau@....com wrote:
> 
> > Somewhat related to this, since our PCIe controller HW is limited to
> > 32-bit AXI address range, before trying to hook up the IOMMU I have
> > tried to limit the dma_mask for PCI cards to DMA_BIT_MASK(32). The
> > reason being that Linux uses a 1 to 1 mapping between PCI addresses
> > and cpu (phys) addresses when there isn't an IOMMU involved, so I
> > think that we need to limit the PCI address space used.
> 
> 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.
> 
> 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.

I think we are missing one crucial bit of infrastructure on ARM64 at
the moment: the dma_set_mask() function should fail if a driver asks
for a mask that is larger than the dma-ranges property of the parent
device (or any device higher up in the hierarchy) allows.

Drivers that want a larger mask should try that first, and then fall
back to a 32-bit mask, which is guaranteed to work.

	Arnd
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