From: Sven Peter via iommu <iommu@lists.linux-foundation.org>
To: "Robin Murphy" <robin.murphy@arm.com>, iommu@lists.linux-foundation.org
Cc: Arnd Bergmann <arnd@kernel.org>,
devicetree@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
Hector Martin <marcan@marcan.st>,
linux-kernel@vger.kernel.org, Rob Herring <robh+dt@kernel.org>,
Mohamed Mediouni <mohamed.mediouni@caramail.com>,
Stan Skowronek <stan@corellium.com>,
Will Deacon <will@kernel.org>,
linux-arm-kernel@lists.infradead.org,
Mark Kettenis <mark.kettenis@xs4all.nl>
Subject: Re: [PATCH 0/3] Apple M1 DART IOMMU driver
Date: Thu, 25 Mar 2021 08:58:16 +0100 [thread overview]
Message-ID: <b05337f3-a4bf-4add-a68d-b9f5c9b8b10d@www.fastmail.com> (raw)
In-Reply-To: <9b9d771a-f6d4-2d27-7516-f5b8315909ed@arm.com>
Hi Robin,
On Wed, Mar 24, 2021, at 16:29, Robin Murphy wrote:
> On 2021-03-20 15:19, Sven Peter wrote:
> >
> > I have just noticed today though that at least the USB DWC3 controller in host
> > mode uses *two* darts at the same time. I'm not sure yet which parts seem to
> > require which DART instance.
> >
> > This means that we might need to support devices attached to two iommus
> > simultaneously and just create the same iova mappings. Currently this only
> > seems to be required for USB according to Apple's Device Tree.
> >
> > I see two options for this and would like to get feedback before
> > I implement either one:
> >
> > 1) Change #iommu-cells = <1>; to #iommu-cells = <2>; and use the first cell
> > to identify the DART and the second one to identify the master.
> > The DART DT node would then also take two register ranges that would
> > correspond to the two DARTs. Both instances use the same IRQ and the
> > same clocks according to Apple's device tree and my experiments.
> > This would keep a single device node and the DART driver would then
> > simply map iovas in both DARTs if required.
>
> This is broadly similar to the approach used by rockchip-iommu and the
> special arm-smmu-nvidia implementation, where there are multiple
> instances which require programming identically, that are abstracted
> behind a single "device". Your case is a little different since you're
> not programming both *entirely* identically, although maybe that's a
> possibility if each respective ID isn't used by anything else on the
> "other" DART?
That would be possible. The only difference is that I need to
program ID 0 of the first DART and ID 1 of the second one. Both
of these IDs are only connected to the same USB controller.
>
> Overall I tend to view this approach as a bit of a hack because it's not
> really describing the hardware truthfully - just because two distinct
> functional blocks have their IRQ lines wired together doesn't suddenly
> make them a single monolithic block with multiple interfaces - and tends
> to be done for the sake of making the driver implementation easier in
> terms of the Linux IOMMU API (which, note, hasn't evolved all that far
> from its PCI-centric origins and isn't exactly great for arbitrary SoC
> topologies).
Yes, the easier driver implementation was my reason to favour this option.
>
> > 2) Keep #iommu-cells as-is but support
> > iommus = <&usb_dart1a 1>, <&usb_dart1b 0>;
> > instead.
> > This would then require two devices nodes for the two DART instances and
> > some housekeeping in the DART driver to support mapping iovas in both
> > DARTs.
> > I believe omap-iommu.c supports this setup but I will have to read
> > more code to understand the details there and figure out how to implement
> > this in a sane way.
>
> This approach is arguably the most honest, and more robust in terms of
> making fewer assumptions, and is used by at least exynos-iommu and
> omap-iommu. In Linux it currently takes a little bit more housekeeping
> to keep track of linked instances within the driver since the IOMMU API
> holds the notion that any given client device is associated with "an
> IOMMU", but that's always free to change at any time, unlike the design
> of a DT binding.
Sounds good. I'll read those drivers and give it a try for v2.
Thanks,
Sven
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prev parent reply other threads:[~2021-03-25 7:59 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-20 15:19 [PATCH 0/3] Apple M1 DART IOMMU driver Sven Peter via iommu
2021-03-20 15:19 ` [PATCH 1/3] iommu: io-pgtable: add DART pagetable format Sven Peter via iommu
2021-03-24 16:37 ` Robin Murphy
2021-03-25 20:47 ` Sven Peter via iommu
2021-03-20 15:20 ` [PATCH 2/3] dt-bindings: iommu: add DART iommu bindings Sven Peter via iommu
2021-03-22 0:15 ` Rob Herring
2021-03-22 18:16 ` Sven Peter via iommu
2021-03-21 16:00 ` [PATCH 0/3] Apple M1 DART IOMMU driver Mark Kettenis
2021-03-21 17:22 ` Sven Peter via iommu
2021-03-21 18:35 ` Mark Kettenis
2021-03-22 22:17 ` Sven Peter via iommu
2021-03-23 20:00 ` Mark Kettenis
2021-03-23 21:03 ` Sven Peter via iommu
2021-03-21 17:28 ` Sven Peter via iommu
2021-03-23 20:53 ` Rob Herring
2021-03-23 22:33 ` Mark Kettenis
2021-03-25 7:53 ` Sven Peter via iommu
2021-03-25 11:50 ` Robin Murphy
2021-03-25 20:49 ` Sven Peter via iommu
2021-03-27 15:33 ` Sven Peter via iommu
2021-03-25 21:41 ` Arnd Bergmann
2021-03-26 15:59 ` Mark Kettenis
2021-03-26 16:09 ` Arnd Bergmann
2021-03-26 16:10 ` Sven Peter via iommu
2021-03-26 16:38 ` Arnd Bergmann
2021-03-26 17:06 ` Sven Peter via iommu
2021-03-26 17:26 ` Mark Kettenis
2021-03-26 17:34 ` Robin Murphy
2021-03-26 17:51 ` Sven Peter via iommu
2021-03-26 19:59 ` Arnd Bergmann
2021-03-26 21:16 ` Mark Kettenis
2021-03-27 15:30 ` Sven Peter via iommu
2021-03-26 20:03 ` Arnd Bergmann
2021-03-26 21:13 ` Mark Kettenis
2021-03-24 15:29 ` Robin Murphy
2021-03-25 7:58 ` Sven Peter via iommu [this message]
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