linux-kernel - Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

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Message-ID: <20140604164132.GF6644@arm.com>
Date:	Wed, 4 Jun 2014 17:41:32 +0100
From:	Will Deacon <will.deacon@....com>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	Dave P Martin <Dave.Martin@....com>, Arnd Bergmann <arnd@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Mark Rutland <Mark.Rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-samsung-soc@...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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Linux IOMMU <iommu@...ts.linux-foundation.org>,
	Rob Herring <robh+dt@...nel.org>,
	Kumar Gala <galak@...eaurora.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Cho KyongHo <pullip.cho@...sung.com>,
	Hiroshi Doyu <hdoyu@...dia.com>
Subject: Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

On Wed, Jun 04, 2014 at 03:35:10PM +0100, Thierry Reding wrote:
> On Mon, Jun 02, 2014 at 11:41:04AM +0100, Dave Martin wrote:
> > In the strictest sense, no.
> > 
> > But for a large set of sane configurations, this probably works.
> > 
> > Small sets of randomly-assigned IDs can just be enumerated one by one.
> > 
> > We wouldn't be able to describe folding and bit shuffling, but we
> > probably don't want to encourage that anyway.
> 
> I'm having some difficulty understanding this. You make it sound like
> there's a fairly arbitrary number of IDs that the SMMU can handle. So
> how is the mapping to devices defined? If you say encourage that does
> make it sound like the assignment of IDs is purely defined by some
> mechanism in software rather than in hardware. Or they are more or less
> randomly picked by someone. If that's the case, is that not something
> that should be dynamically allocated by the kernel rather than put into
> the device tree?

The set of StreamIDs that can be generated by a master is fixed in the
hardware. The SMMU can then be programmed to map these incoming IDs onto
a context ID (or a set of context IDs), which are the IDs used internally
by the SMMU to find the page tables etc.

The StreamID -> ContextID mapping is dynamic and controlled by software.
The Master -> StreamIDs mapping is fixed in the hardware.

Will

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