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

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Date:	Wed, 25 Jun 2014 11:14:09 +0100
From:	Will Deacon <will.deacon@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Olav Haugan <ohaugan@...eaurora.org>,
	Rob Herring <robherring2@...il.com>,
	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>,
	Thierry Reding <thierry.reding@...il.com>,
	Kumar Gala <galak@...eaurora.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	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>
Subject: Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

On Wed, Jun 25, 2014 at 11:12:13AM +0100, Arnd Bergmann wrote:
> On Wednesday 25 June 2014 10:57:36 Will Deacon wrote:
> > So far, I've been avoiding the hardcoding. However, you could potentially
> > build a system with a small number of SMRs (compared to the number of
> > StreamIDs) and allocate the StreamIDs in such a way that I think the dynamic
> > configuration would be NP complete if we require an optimal SMR allocation.
> > 
> > However:
> > 
> >   (1) I don't know of a system where this is the case
> >   (2) Not much work has been done on improving the dynamic allocator yet
> > 
> > which is why I'm still favouring dynamic configuration in the driver.
> > 
> > The other thing I forgot to mention earlier is that we need to support
> > device hotplug in the future, so some level of dynamic configuration
> > will always be required.
> 
> Ok, got it. So we just hope that we can make dynamic configuration
> work all the time, but if it all fails, then we come up with a
> hardcoded configuration method.
> 
> I guess this could be done similarly to how we handle clocks on
> a lot of systems: generally these are dynamic, but you have the
> option to provide hints in the clock controller node about how
> you expect things to be configured.
> 
> For the SMMU that could mean that (if we get into the situation you
> describe), we add optional properties to the SMMU node itself
> describing how we expect the SMRs to be used.

That sounds good to me! Thanks for the discussion.

Will
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