linux-kernel - Re: [PATCH] devicetree: Add generic IOMMU device tree bindings

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Date:	Wed, 21 May 2014 10:50:38 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	Dave Martin <Dave.Martin@....com>,
	linux-arm-kernel@...ts.infradead.org,
	Mark Rutland <mark.rutland@....com>,
	devicetree@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Grant Grundler <grundler@...omium.org>,
	Joerg Roedel <joro@...tes.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
	Marc Zyngier <marc.zyngier@....com>,
	iommu@...ts.linux-foundation.org,
	Kumar Gala <galak@...eaurora.org>, linux-tegra@...r.kernel.org,
	Cho KyongHo <pullip.cho@...sung.com>
Subject: Re: [PATCH] devicetree: Add generic IOMMU device tree bindings

On Wednesday 21 May 2014 10:26:11 Thierry Reding wrote:
> On Tue, May 20, 2014 at 10:26:12PM +0200, Arnd Bergmann wrote:
> > On Tuesday 20 May 2014 16:24:59 Dave Martin wrote:
> > > On Tue, May 20, 2014 at 02:41:18PM +0200, Arnd Bergmann wrote:
> > > > On Tuesday 20 May 2014 14:02:43 Thierry Reding wrote:
> [...]
> > > > > Multiple-master IOMMU:
> > > > > ----------------------
> > > > > 
> > > > > 	iommu {
> > > > > 		/* the specifier represents the ID of the master */
> > > > > 		#address-cells = <1>;
> > > > > 		#size-cells = <0>;
> > > 
> > > How do we know the size of the input address to the IOMMU?  Do we
> > > get cases for example where the IOMMU only accepts a 32-bit input
> > > address, but some 64-bit capable masters are connected through it?
> > 
> > I was stuck on this question for a while before, but then I realized
> > that it doesn't matter at all: It's the IOMMU driver itself that
> > manages the address space, and it doesn't matter if a slave can
> > address a larger range than the IOMMU can accept. If the IOMMU
> > needs to deal with the opposite case (64-bit input addresses
> > but a 32-bit master), that limitation can be put into the specifier.
> 
> Isn't this what DMA masks are for? Couldn't the IOMMU simply use the
> master device's DMA mask to do the right thing here?

Ah, yes. I guess that's the right way to do it.

> > > For determining dma masks, it is the output address that it
> > > important.  Santosh's code can probably be taught to handle this,
> > > if given an additional traversal rule for following "iommus"
> > > properties.  However, deploying an IOMMU whose output address size
> > > is smaller than the 
> > 
> > Something seems to be missing here. I don't think we want to handle
> > the case where the IOMMU output cannot the entire memory address
> > space. If necessary, that would mean using both an IOMMU driver
> > and swiotlb, but I think it's a reasonable assumption that hardware
> > isn't /that/ crazy.
> 
> Similarily, should the IOMMU not be treated like any other device here?
> Its DMA mask should determine what address range it can access.

Right. But for that we need a dma-ranges property in the parent of the
iommu, just so the mask can be set correctly and we don't have to
rely on the 32-bit fallback case.

	Arnd
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