linux-kernel - Re: [PATCH v3 02/10] devicetree: Add generic IOMMU device tree bindings

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Date:	Mon, 30 Jun 2014 16:24:59 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Thierry Reding <thierry.reding@...il.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>,
	Arnd Bergmann <arnd@...db.de>,
	Will Deacon <will.deacon@....com>,
	Joerg Roedel <joro@...tes.org>
CC:	Cho KyongHo <pullip.cho@...sung.com>,
	Grant Grundler <grundler@...omium.org>,
	Dave Martin <Dave.Martin@....com>,
	Marc Zyngier <marc.zyngier@....com>,
	Hiroshi Doyu <hdoyu@...dia.com>,
	Olav Haugan <ohaugan@...eaurora.org>,
	Paul Walmsley <pwalmsley@...dia.com>,
	Rhyland Klein <rklein@...dia.com>,
	Allen Martin <amartin@...dia.com>, devicetree@...r.kernel.org,
	iommu@...ts.linux-foundation.org,
	linux-arm-kernel@...ts.infradead.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 02/10] devicetree: Add generic IOMMU device tree bindings

On 06/26/2014 02:49 PM, Thierry Reding wrote:
> From: Thierry Reding <treding@...dia.com>
> 
> This commit introduces a generic device tree binding for IOMMU devices.
> Only a very minimal subset is described here, but it is enough to cover
> the requirements of both the Exynos System MMU and Tegra SMMU as
> discussed here:
> 
>     https://lkml.org/lkml/2014/4/27/346

> diff --git a/Documentation/devicetree/bindings/iommu/iommu.txt b/Documentation/devicetree/bindings/iommu/iommu.txt

> +When an "iommus" property is specified in a device tree node, the IOMMU will
> +be used for address translation. If a "dma-ranges" property exists in the
> +device's parent node it will be ignored. An exception to this rule is if the
> +referenced IOMMU is disabled, in which case the "dma-ranges" property of the
> +parent shall take effect.

I wonder how useful that paragraph is. The fact that someone disabled a
particular IOMMU's node doesn't necessarily mean that the HW can
actually do that; an IOMMU might always be active in HW and always
translate accesses by some master. In that case, the fallback to
dma-ranges wouldn't correlate with what the HW actually does. Perhaps
all we need is to add a note to that effect here?
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