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

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

Hi Thierry,

On Thu, Jun 26, 2014 at 09:49:42PM +0100, Thierry Reding wrote:
> From: Thierry Reding <treding-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> 
> 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
> 
> Signed-off-by: Thierry Reding <treding-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>

[...]

> +Required properties:
> +--------------------
> +- #iommu-cells: The number of cells in an IOMMU specifier needed to encode an
> +  address.
> +
> +Typical values for the above include:
> +- #iommu-cells = <0>: Single master IOMMU devices are not configurable and
> +  therefore no additional information needs to be encoded in the specifier.
> +  This may also apply to multiple master IOMMU devices that do not allow the
> +  association of masters to be configured.

A multiple-master capable IOMMU could be built with a single master, but
we'd still need #iommu-cells > 0 here. I appreciate this is just an example,
but the wording sounds like it's enforced.

> +- #iommu-cells = <1>: Multiple master IOMMU devices may need to be configured
> +  in order to enable translation for a given master. In such cases the single
> +  address cell corresponds to the master device's ID.

Again, we will definitely need more than one cell in this case, as I fully
expect multiple StreamIDs for each master (e.g. Qualcomm mentioned on the
list the other day that they have a master emitting 43 unique IDs).

Anyway, the actual binding looks great, I just don't want people to think
they need to do something different because they don't fit your example
use-cases.

> +Multiple-master IOMMU:
> +----------------------
> +
> +	iommu {
> +		/* the specifier represents the ID of the master */
> +		#iommu-cells = <1>;
> +	};
> +
> +	master {
> +		/* device has master ID 42 in the IOMMU */
> +		iommus = <&/iommu 42>;
> +	};
> +
> +Multiple-master IOMMU with configurable DMA window:
> +---------------------------------------------------
> +
> +	/ {
> +		#address-cells = <1>;
> +		#size-cells = <1>;
> +
> +		iommu {
> +			/* master ID, address and length of DMA window */
> +			#iommu-cells = <4>;
> +		};
> +
> +		master {
> +			/* master ID 42, 4 GiB DMA window starting at 0 */
> +			iommus = <&/iommu  42  0  0x1 0x0>;
> +		};
> +	};

Could you also please include an example of a master with multiple IDs?

Will