linux-kernel - Re: [PATCH v4] devicetree: Add generic IOMMU device tree bindings

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Date:	Thu, 31 Jul 2014 11:50:17 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	Olof Johansson <olof@...om.net>, Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Arnd Bergmann <arnd@...db.de>,
	Will Deacon <Will.Deacon@....com>,
	Joerg Roedel <joro@...tes.org>,
	Cho KyongHo <pullip.cho@...sung.com>,
	Grant Grundler <grundler@...omium.org>,
	Dave P Martin <Dave.Martin@....com>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Hiroshi Doyu <hdoyu@...dia.com>,
	Olav Haugan <ohaugan@...eaurora.org>,
	Varun Sethi <varun.sethi@...escale.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] devicetree: Add generic IOMMU device tree bindings

On Thu, Jul 31, 2014 at 11:09:06AM +0100, Thierry Reding wrote:
> On Wed, Jul 30, 2014 at 07:18:42PM +0100, Mark Rutland wrote:
> [...]
> > > >> +
> > > >> +Multiple-master IOMMU with configurable DMA window:
> > > >> +---------------------------------------------------
> > > >> +
> > > >> +     / {
> > > >> +             #address-cells = <1>;
> > > >> +             #size-cells = <1>;
> > > >> +
> > > >> +             iommu {
> > > >> +                     /* master ID, address and length of DMA window */
> > > >> +                     #iommu-cells = <4>;
> > > >> +             };
> > > >> +
> > > >> +             master {
> > > >> +                     /* master ID 42, 4 GiB DMA window starting at 0 */
> > > >> +                     iommus = <&/iommu  42  0  0x1 0x0>;
> > > >
> > > > Is this that window is from the POV of the master, i.e. the master can
> > > > address 0x0 to 0xffffffff when generating transactions, and these get
> > > > translated somehow?
> > > >
> > > > Or is this the physical addresses to allocate to the master?
> > > 
> > > It needs to be clarified in the documentation, but as far as I know it
> > > is the DMA address space that is used.
> > 
> > Ok. So that's pre-translation, from the POV of the master?
> 
> Correct. It represents the window of the IOMMU's addressable I/O virtual
> address space that should be assigned to this particular master.
> 
> > If we don't have that knowledge about the master already (e.g. based on
> > the compatible string), surely we always need that information in a
> > given iommu-specifier format? Otherwise certain iommus won't be able to
> > handle masters with limited addressing only due to limitations of their
> > binding.
> 
> This is only used for what's often called a windowed IOMMU. Many IOMMUs
> (non-windowed) typically allow only a complete address space to be
> assigned to a master without additional control over subregions. So this
> is really a property/capability of the IOMMU rather than the masters
> themselves.

I'm not sure I follow, but I'm happy to wait until we have the first
windowed IOMMU using this binding. I'll try to get myself up to speed in
the mean time.

> There are already other means to respect the addressing limitations of
> masters. We typcially use a device's DMA mask for this, and it's natural
> to reuse that for I/O virtual addresses since they will in fact take the
> place of physical addresses for the master when translation is enabled.

Ok, that covers my worry then.

Cheers,
Mark.
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