RE: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

Stuart Yoder
2014-06-17T14:39:35Z

> -----Original Message-----
> From: Sethi Varun-B16395
> Sent: Tuesday, June 17, 2014 5:27 AM
> To: Yoder Stuart-B08248; Will Deacon
> Cc: Thierry Reding; Mark Rutland; devicetree@vger.kernel.org; linux-
> samsung-soc@vger.kernel.org; Pawel Moll; Arnd Bergmann; Ian Campbell;
> Grant Grundler; Stephen Warren; linux-kernel@vger.kernel.org; Marc
> Zyngier; Linux IOMMU; Rob Herring; Kumar Gala; linux-
> tegra@vger.kernel.org; Cho KyongHo; Dave P Martin; linux-arm-
> kernel@lists.infradead.org
> Subject: RE: [PATCH v2] devicetree: Add generic IOMMU device tree
> bindings
>
>
>
> > -----Original Message-----
> > From: Yoder Stuart-B08248
> > Sent: Tuesday, June 17, 2014 12:24 AM
> > To: Will Deacon
> > Cc: Sethi Varun-B16395; Thierry Reding; Mark Rutland;
> > devicetree@vger.kernel.org; linux-samsung-soc@vger.kernel.org; Pawel
> > Moll; Arnd Bergmann; Ian Campbell; Grant Grundler; Stephen Warren;
> linux-
> > kernel@vger.kernel.org; Marc Zyngier; Linux IOMMU; Rob Herring; Kumar
> > Gala; linux-tegra@vger.kernel.org; Cho KyongHo; Dave P Martin; linux-
> arm-
> > kernel@lists.infradead.org
> > Subject: RE: [PATCH v2] devicetree: Add generic IOMMU device tree
> > bindings
> >
> >
> >
> > > -----Original Message-----
> > > From: Will Deacon [mailto:will.deacon@arm.com]
> > > Sent: Monday, June 16, 2014 12:04 PM
> > > To: Yoder Stuart-B08248
> > > Cc: Sethi Varun-B16395; Thierry Reding; Mark Rutland;
> > > devicetree@vger.kernel.org; linux-samsung-soc@vger.kernel.org; Pawel
> > > Moll; Arnd Bergmann; Ian Campbell; Grant Grundler; Stephen Warren;
> > > linux- kernel@vger.kernel.org; Marc Zyngier; Linux IOMMU; Rob
> Herring;
> > > Kumar Gala; linux-tegra@vger.kernel.org; Cho KyongHo; Dave P Martin;
> > > linux-arm- kernel@lists.infradead.org
> > > Subject: Re: [PATCH v2] devicetree: Add generic IOMMU device tree
> > > bindings
> > >
> > > Hi Stuart,
> > >
> > > On Mon, Jun 16, 2014 at 05:56:32PM +0100, Stuart Yoder wrote:
> > > > > Do you have use-cases where you really need to change these
> > > > > mappings dynamically?
> > > >
> > > > Yes. In the case of a PCI bus-- you may not know in advance how
> many
> > > > PCI devices there are until you probe the bus. We have another
> FSL
> > > > proprietary bus we call the "fsl-mc" bus that is similar.
> > >
> > > For that case, though, you could still describe an algorithmic
> > > transformation from RequesterID to StreamID which corresponds to a
> > > fixed mapping.
> > >
> > > > Another thing to consider-- starting with SMMUv2, as you know,
> there
> > > > is a new distributed architecture with multiple TBUs and a
> > > > centralized TCU that walks the SMMU page tables. So instead of
> > > > sprinkling multiple SMMUs all over an SoC you now have the option a
> > > > 1 central TCU and
> > > sprinkling
> > > > multiple TBUs around. However, this means that the stream ID
> > > namespace
> > > > is now global and can be pretty limited. In the SMMU
> implementation
> > > > we have there are only 64 stream ID total for our Soc. But we have
> > > > many
> > > more
> > > > masters than that.
> > > >
> > > > So we look at stream IDs as really corresponding to an 'isolation
> > > context'
> > > > and not to a bus master. An isolation context is the domain you
> are
> > > > trying to isolate with the SMMU. Devices that all belong to the
> > > > same 'isolation context' can share the same stream ID, since they
> > > > share the same domain and page tables.
> > >
> > > Ok, this is more compelling.
> > >
> > > > So, perhaps by default some/most SMMU masters may have a default
> > > > stream
> > > ID
> > > > of 0x0 that is used by the host...and that could be represented
> > > > statically in the device tree.
> > > >
> > > > But, we absolutely will need to dynamically set new stream IDs into
> > > > masters when a new IOMMU 'domain' is created and devices
> > > > are added to it. All the devices in a domain will share
> > > > the same stream ID.
> > > >
> > > > So whatever we do, let's please have an architecture flexible
> enough
> > > > to allow for this.
> > >
> > > What is the software interface to the logic that assigns the
> StreamIDs?
> > > Is
> > > it part of the SMMU, or a separate device (or set of devices)?
> >
> > For us at the hardware level there are a few different ways that the
> > streamIDs can be set. It is not part of the SMMU. In the cases where
> > there is simply
> > 1 device to 1 streamID (e.g. USB controller) there is an SoC register
> > where
> > you just program the stream ID. In the case of PCI, our PCI
> controller
> > has a RequesterID-to-streamID table that you set via some PCI
> controller
> > registers.
> >
> > The way we generally thought it would work was something like
> > this:
> > -u-boot/bootloader makes any static streamID allocation if needed,
> > sets a default streamID (e.g. 0x0) in device and expresses
> > that in the device tree
> > -device tree would express relationship between devices
> > (including bus controllers) and the SMMU through mmu-masters
> > property
> > -u-boot would express the range of unused (or used) streamIDs via a
> > new
> > device tree property so the kernel SMMU driver knows what streamIDs
> > are
> > free
> > -in the SMMU driver a different vendor specific 'add_device'
> callback
> > could be used to handle our special cases where we need to
> set/change
> > the stream ID for devices added to a domain
>
> Another possibility, could be to program the stream Id in the device
> registers (reference for the stream ID register can be obtained from the
> device tree) during device attach. This could be relevant in case of
> VFIO, when we are assigning multiple devices to a single VM. All the
> devices can share the same stream ID.

Actually, that is what I meant-- do the special case handling during
device "attach" (not add).

Stuart