Re: [PATCH] devicetree: Add generic IOMMU device tree bindings

[Cho KyongHo]

On Fri, 16 May 2014 14:23:18 +0200, Thierry Reding wrote:
> From: Thierry Reding <treding@xxxxxxxxxx>
> 
> 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
> 
> More advanced functionality such as the dma-ranges property can easily
> be added in a backwards-compatible way. In the absence of a dma-ranges
> property it should be safe to default to the whole address space.
> 
> Signed-off-by: Thierry Reding <treding@xxxxxxxxxx>
> ---
>  Documentation/devicetree/bindings/iommu/iommu.txt | 109 ++++++++++++++++++++++
>  1 file changed, 109 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/iommu/iommu.txt
> 
> diff --git a/Documentation/devicetree/bindings/iommu/iommu.txt b/Documentation/devicetree/bindings/iommu/iommu.txt
> new file mode 100644
> index 000000000000..2d67b52b656e
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/iommu/iommu.txt
> @@ -0,0 +1,109 @@
> +This document describes the generic device tree binding for IOMMUs and their
> +master(s).
> +
> +
> +IOMMU device node:
> +==================
> +
> +An IOMMU can provide the following services:
> +
> +* Remap address space to allow devices to access physical memory ranges that
> +  they otherwise wouldn't be capable of accessing.
> +
> +  Example: 32-bit DMA to 64-bit physical addresses
> +
> +* Implement scatter-gather at page level granularity so that the device does
> +  not have to.
> +
> +* Provide system protection against "rogue" DMA by forcing all accesses to go
> +  through the IOMMU and faulting when encountering accesses to unmapped
> +  address regions.
> +
> +* Provide address space isolation between multiple contexts.
> +
> +  Example: Virtualization
> +
> +Device nodes compatible with this binding represent hardware with some of the
> +above capabilities.
> +
> +IOMMUs can be single-master or multiple-master. Single-master IOMMU devices
> +typically have a fixed association to the master device, whereas multiple-
> +master IOMMU devices can translate accesses from more than one master.
> +
> +Required properties:
> +--------------------
> +- #iommu-cells: The number of cells in an IOMMU specifier. The meaning of the
> +  cells is defined by the binding for the IOMMU device.
> +
> +  Typical values include:
> +  * 0: Single-master IOMMU devices are often not configurable, therefore the
> +    specifying doesn't need to encode any information and can be empty.
> +
> +  * 1: Multiple-master IOMMU devices need to know for which master they should
> +    enable translation. Typically the single cell in the specifier corresponds
> +    to the master device's ID.
> +
> +
> +IOMMU master node:
> +==================
> +
> +Devices that access memory through an IOMMU are called masters. A device can
> +have multiple master interfaces (to one or more IOMMU devices).
> +
> +Required properties:
> +--------------------
> +- iommus: A list of phandle and IOMMU specifier pairs that describe the IOMMU
> +  master interfaces of the device. One entry in the list describes one master
> +  interface of the device.
> +
> +Optional properties:
> +--------------------
> +- iommu-names: A list of names identifying each entry in the iommus property.
> +
> +
> +Examples:
> +=========
> +
> +Single-master IOMMU:
> +--------------------
> +
> +	iommu {
> +		#iommu-cells = <0>;
> +	};
> +
> +	master {
> +		iommu = <&/iommu>;
> +	};
> +

Great work, Thierry.

One simple comment.

This should be also applicable to multi-master IOMMUs that the masters
of an IOMMU is not configurable with ID or something.
I think the title needs to be changed to cover such IOMMUs which always
translate master's transactions and unable to change the configuration
of the relationship between the masters and IOMMUs by S/W.

Regards,

KyongHo

> +Multi-master IOMMU:
> +-------------------
> +
> +	iommu {
> +		/* the specifier represents the ID of the master */
> +		#iommu-cells = <1>;
> +	};
> +
> +	master {
> +		/* device has master ID 42 in the IOMMU */
> +		iommu = <&/iommu 42>;
> +	};
> +
> +Multi-master device:
> +--------------------
> +
> +	/* single-master IOMMU */
> +	iommu@1 {
> +		#iommu-cells = <0>;
> +	};
> +
> +	/* multi-master IOMMU */
> +	iommu@2 {
> +		/* the specifier represents the ID of the master */
> +		#iommu-cells = <1>;
> +	};
> +
> +	/* device with two master interfaces */
> +	master {
> +		iommus = <&/iommu@1>,    /* master of the single-master IOMMU */
> +			 <&/iommu@2 42>; /* ID 42 in multi-master IOMMU */
> +	};
> -- 
> 1.9.2
> 
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