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

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Date:	Mon, 14 Jul 2014 06:13:03 -0400
From:	Rob Clark <robdclark@...il.com>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	Arnd Bergmann <arnd@...db.de>, Will Deacon <will.deacon@....com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	Mark Rutland <Mark.Rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Joerg Roedel <joro@...tes.org>,
	Olav Haugan <ohaugan@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Grant Grundler <grundler@...omium.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Varun Sethi <varun.sethi@...escale.com>,
	Cho KyongHo <pullip.cho@...sung.com>,
	Dave P Martin <Dave.Martin@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Hiroshi Doyu <hdoyu@...dia.com>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH v4] devicetree: Add generic IOMMU device tree bindings

On Mon, Jul 14, 2014 at 2:24 AM, Thierry Reding
<thierry.reding@...il.com> wrote:
> On Sat, Jul 12, 2014 at 08:57:31AM -0400, Rob Clark wrote:
>> On Sat, Jul 12, 2014 at 8:22 AM, Arnd Bergmann <arnd@...db.de> wrote:
> [...]
>> > The way that Thierry's binding does that is the obvious solution to this,
>> > and it mirrors what we do in practically every other subsystem. I definitely
>> > want the SMMU to change before anybody starts using it in a real system,
>> > which we fortunately do not have yet.
>>
>> hmm, well if some of the things I need for (like this or batching
>> mappings) are too weird and gpu specific, I'm willing to duplicate the
>> IOMMU driver in drm/msm.  It really isn't so much code, and that gives
>> me a lot more more flexibility to do crazy things... at some point I'm
>> probably going to want to do context switches by banging the IOMMU
>> registers directly from the gpu.
>
> If the IOMMU API doesn't provide for what you need, then perhaps it's
> time to enhance it? We do that all the time in other parts of the
> kernel, why should IOMMU be special?

sure.. and my comment was also about the map/unmap batching.

Bypassing IOMMU wouldn't be my first choice.  (Especially because I'd
then get to implement it twice.)  But if some of the things I need are
too specific to one driver (or worse, problematic for other IOMMU
use-cases which I don't know about), then it is an option I'd be
willing to consider.  If nothing else, it would get me out of
allocating sglists for every buffer..  I wonder how much memory
scatterlists take up for 500M of gfx buffers?

> It seems to me like context switching for per-process address space
> isolation is one of the important features of an IOMMU. If the current
> API doesn't let you do that then we should think of ways how it can be
> improved. And if it doesn't do it fast enough, then we should equally
> find ways to speed it up.
>
> This is part of why I think it would be good to have explicit objects
> associated with IOMMU contexts. That would give us a good place to add
> caching for this kind of situation. Currently we're required to handle
> most of this in drivers (map from struct device to context, ...).

well, it is at least awkward that the current api conflates attaching
device and attaching context.  I think we could get some use out of an
iommu_swap() API which conceptually acts as:

  iommu_swap(olddomain, newdomain, dev)
  {
     iommu_detach_device(olddomain, dev);
     iommu_attach_device(newdomain, dev);
  }

BR,
-R

> Thierry
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