linux-kernel - Re: [PATCH 2/2] ARM: IOMMU: Tegra30: Add iommu

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Message-ID: <20120124142521.GE6269@8bytes.org>
Date:	Tue, 24 Jan 2012 15:25:21 +0100
From:	"joro@...tes.org" <joro@...tes.org>
To:	Felipe Balbi <balbi@...com>
Cc:	Hiroshi Doyu <hdoyu@...dia.com>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-mm-sig-bounces@...ts.linaro.org" 
	<linaro-mm-sig-bounces@...ts.linaro.org>
Subject: Re: [PATCH 2/2] ARM: IOMMU: Tegra30: Add iommu_ops for SMMU driver

On Tue, Jan 24, 2012 at 03:46:01PM +0200, Felipe Balbi wrote:
> On Tue, Jan 24, 2012 at 02:41:21PM +0100, Hiroshi Doyu wrote:
> > Actually I really like the concept of this "domain" now, which hides
> > the H/W hierarchy from users.
> > 
> > But in Tegra SMMU/GART case, there's a single one IOMMU device in the
> > system. Keeping a iommu device list in a domain and iterating iommu
> > device list in each iommu_ops seem to be so nice, but I'm afraid that
> > this may be a bit too much when one already knows that there's only
> > one IOMMU device in the system.
> >
> > If there's no actual problem for 1-1 mapping between IOMMU H/Ws and
> > domains, I think that it may not so bad to keep the original code(1-1)
> > for GART and SMMU. What do you think?
> 
> I think it boils down to "extensability". If you can truly/fully
> guarantee that there will *always* be a single IOMMU on all upcoming
> Tegras, then it's really overkill.
> 
> But if there's even a remote possibility of the HW being changed and you
> end up with more IOMMUs, things start to feel necessary for the sake of
> making it easy to extend.

Right. But I am fine with the logic as-is when there is only one SMMU in
the system. But please also change the IOMMU driver so that it really
only initializes a single SMMU. When boards pop up with more than one
you we notice that assumption in the code again and are reminded to
change it.


	Joerg

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