Re: [PATCH v4] devicetree: Add generic IOMMU device tree bindings

Mark Rutland
2014-07-30T15:26:53Z

Hi Thierry,

This looks sane to me.

I just have a few questions below which are hopefully simple/stupid.

On Fri, Jul 04, 2014 at 04:29:17PM +0100, Thierry Reding wrote:
> From: Thierry Reding <treding@nvidia.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
>
> Signed-off-by: Thierry Reding <treding@nvidia.com>
> ---
> Changes in v4:
> - clarify that disabling an IOMMU DT node may not disable translation
> - be more explicit that examples are only examples
> - add multi-ID master example
>
> Changes in v3:
> - use #iommu-cells instead of #address-cells/#size-cells
> - drop optional iommu-names property
>
> Changes in v2:
> - add notes about "dma-ranges" property (drop note from commit message)
> - document priorities of "iommus" property vs. "dma-ranges" property
> - drop #iommu-cells in favour of #address-cells and #size-cells
> - remove multiple-master device example
>
> Documentation/devicetree/bindings/iommu/iommu.txt | 172 ++++++++++++++++++++++
> 1 file changed, 172 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..464a81eaaf61
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/iommu/iommu.txt
> @@ -0,0 +1,172 @@
> +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.
> +
> +The device tree node of the IOMMU device's parent bus must contain a valid
> +"dma-ranges" property that describes how the physical address space of the
> +IOMMU maps to memory. An empty "dma-ranges" property means that there is a
> +1:1 mapping from IOMMU to memory.
> +
> +Required properties:
> +--------------------
> +- #iommu-cells: The number of cells in an IOMMU specifier needed to encode an
> + address.
> +
> +The meaning of the IOMMU specifier is defined by the device tree binding of
> +the specific IOMMU. Below are a few examples of typical use-cases:
> +
> +- #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. Note that an IOMMU can by design
> + be multi-master yet only expose a single master in a given configuration.
> + In such cases the number of cells will usually be 1 as in the next case.
> +- #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. In some cases more than
> + one cell can be required to represent a single master ID.
> +- #iommu-cells = <4>: Some IOMMU devices allow the DMA window for masters to
> + be configured. The first cell of the address in this may contain the master
> + device's ID for example, while the second cell could contain the start of
> + the DMA window for the given device. The length of the DMA window is given
> + by the third and fourth cells.
> +
> +Note that these are merely examples and real-world use-cases may use different
> +definitions to represent their individual needs. Always refer to the specific
> +IOMMU binding for the exact meaning of the cells that make up the specifier.
> +
> +
> +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.
> +
> +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. Note that merely disabling a device tree node does
> +not guarantee that the IOMMU is really disabled since the hardware may not
> +have a means to turn off translation.
> +
> +
> +Notes:
> +======
> +
> +One possible extension to the above is to use an "iommus" property along with
> +a "dma-ranges" property in a bus device node (such as PCI host bridges). This
> +can be useful to describe how children on the bus relate to the IOMMU if they
> +are not explicitly listed in the device tree (e.g. PCI devices). However, the
> +requirements of that use-case haven't been fully determined yet. Implementing
> +this is therefore not recommended without further discussion and extension of
> +this binding.
> +
> +
> +Examples:
> +=========
> +
> +Single-master IOMMU:
> +--------------------
> +
> + iommu {
> + #iommu-cells = <0>;
> + };
> +
> + master {
> + iommus = <&/iommu>;

Nit: this should be iommus = <&{/iommu}>, or it's not valid dts syntax.

> + };
> +
> +Multiple-master IOMMU with fixed associations:
> +----------------------------------------------
> +
> + /* multiple-master IOMMU */
> + iommu {
> + /*
> + * Masters are statically associated with this IOMMU and
> + * address translation is always enabled.
> + */
> + #iommu-cells = <0>;

I don't follow why translation being always enabled is relevant to the
example; that would seem to be independent from the binding.

Surely the key point is that with no way to distinguish devices, they
presumably share the same translations?

> + };
> +
> + /* static association with IOMMU */
> + master@1 {
> + reg = <1>;
> + iommus = <&/iommu>;
> + };
> +
> + /* static association with IOMMU */
> + master@2 {
> + reg = <2>;
> + iommus = <&/iommu>;
> + };
> +
> +Multiple-master IOMMU:
> +----------------------
> +
> + iommu {
> + /* the specifier represents the ID of the master */
> + #iommu-cells = <1>;
> + };
> +
> + master@1 {
> + /* device has master ID 42 in the IOMMU */
> + iommus = <&/iommu 42>;
> + };
> +
> + master@2 {
> + /* device has master IDs 23 and 24 in the IOMMU */
> + iommus = <&/iommu 23>, <&/iommu 24>;
> + };

In future I suspect master will need to be able to identify which master
IDs correspond to which of their master ports (where each port might
have an arbitrary number of master IDs).

While we don't need that for the first run, it would be nice to have
that looked into so master bindings don't come up with arbitrarily
different ways of doing that.

> +
> +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>;

Is this that window is from the POV of the master, i.e. the master can
address 0x0 to 0xffffffff when generating transactions, and these get
translated somehow?

Or is this the physical addresses to allocate to the master?

Cheers,
Mark.