Re: [RFC PATCH 0/3] Introduce iommu-map-masked for platform devices

[Rob Herring]

On Sun, Sep 28, 2025 at 12:17 PM Charan Teja Kalla
<charan.kalla@xxxxxxxxxxxxxxxx> wrote:
>
> This series introduces a new iommu property called iommu-map-masked(may
> be there is a better name), which is used to represent the IOMMU
> specifier pairs for each function of a __multi-functional platform
> device__, where each function can emit unique master id(s) that can be
> associated with individual translation context.
>
> Currently, the iommu configuration - at least for arm architecture-
> requires all the functions of a platform device will be represented
> under single dt node thus endup in using only a single translation
> context.
>
> A simple solution to associate individual translation context for each
> function of a device can be through creating per function child nodes in
> the device tree, but dt is only to just represent the soc layout to
> linux kernel.
>
> Supporting such cases requires a new iommu property called,
> iommu-map-masked(taking cue from iommu-map for pci devices) and syntax
> is:
>    iommu-map-masked = <FUNCTION_ID1 &iommu ID1 MASK1>,
>                       <FUNCTION_ID2 &iommu ID2 MASK2>;
> NOTE: As an RFC, it is considered that this property always expects 4
> cells.
>
> During the probe phase of the driver for a multi-functional device
> behind an IOMMU, a child device is instantiated for each FUNCTION_ID.
> The call to of_dma_configure_id() on each child sets up the IOMMU
> configuration, ensuring that each function of the device is associated
> with a distinct translation context.
>
> This property can also be used in association with 'iommus=' when dt
> bindings requires the presence of 'iommus=', example[2]. For these
> cases, representation will be(on arm64):
>    iommus = <&iommu sid mask>; //for default function.
>    iommu-map-masked = <FUNCTION_ID &iommu sid mask>;//additional
> function.

Where does the FUNCTION_ID value come from?

Why can't you just have multiple "iommus" entries where the index
defines the default and any FUNCTION_ID entries? What's in each index
is specific to the device.

Rob