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

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Date:	Wed, 25 Jun 2014 10:17:02 +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 Tue, Jun 24, 2014 at 07:20:56PM +0100, Arnd Bergmann wrote:
> On Tuesday 24 June 2014 19:11:50 Will Deacon wrote:
> > On Tue, Jun 24, 2014 at 06:57:44PM +0100, Olav Haugan wrote:
> > > We do describe the masked StreamID (SID) but we need to specify the mask
> > > that the SMMU should apply to the incoming SIDs, right?
> > > 
> > > We have a bus master that emits 43 unique SIDs. However, we have only 40
> > > SMMU_SMRn registers in the SMMU. So we need to mask out some of the
> > > incoming SID bits so that the 43 SIDs can match one of 40 entries in the
> > > SMR.
> > 
> > Hmm, so you're talking about stream matching, right? That doesn't belong in
> > the device-tree. I appreciate that the current driver does a terrible job at
> > allocating the SMRs (it's bloody difficult!), but we should try to improve
> > the dynamic behaviour instead of moving configuration of the SMMU out into
> > device-tree, where it's inflexible at best.
> > 
> > There have been patches previously posted by Andreas Herrmann helping here.
> > I'd be glad to see them revived.
> 
> Note that there are areas where we have in the past decided that dynamic
> configuration is just too hard for the kernel to do and that we're better
> off putting the configuration into DT. Pinctrl and clocks are at least
> partially in that category.
> 
> It's always best if you can get the kernel to do things in the ideal
> way where that is possible, but getting there may be just not worth it.
> 
> I have no idea where it should be for SMMU, but it's something to consider:
> if you can take reasonable shortcuts by reading parts of the configuration
> from DT, you may just as well do that.

I treat this in the same manner as the topology bindings we discussed
previously; we should do a best-effort to configure things dynamically and
solve corner-cases and quirks as special cases.

Will
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