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

From: Rob Clark
Date: Fri Jul 11 2014 - 16:55:24 EST

On Fri, Jul 4, 2014 at 11:29 AM, Thierry Reding
<thierry.reding@xxxxxxxxx> 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
>
> Signed-off-by: Thierry Reding <treding@xxxxxxxxxx>
> ---
> 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>;
> + };
> +
> +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>;
> + };
> +
> + /* 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>;
> + };
> +
> +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>;
> + };
> + };


ok, so I was working through this to try to convert my
{qcom,msm}-iommu-v0 RFC over to using these bindings. For background,
I was initially using something that looked a bit more like the
current arm-smmu bindings:

gpu {
#stream-id-cells = <16>;
...
};

gfx3d: qcom,iommu@7c00000 {
compatible = "qcom,iommu-v0";
...
mmu-masters =
/* gfx3d_user: */
<&gpu 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15>,
/* gfx3d_priv: */
<&gpu 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31>;
};

gfx3d1: qcom,iommu@7d00000 {
compatible = "qcom,iommu-v0";
...
mmu-masters =
/* gfx3d_user: */
<&gpu 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15>,
/* gfx3d_priv: */
<&gpu 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31>;
};

With my current arrangement, I have everything I need when the iommu
device is probed to set up each of the context banks

This proposal inverts that relationship. Which forces me to do a lot
more (including DT parsing) on device attach. Which I'm not a huge
fan of. Ie. if I even wanted to try to implement per-process
pagetables for gpu without completely going behind the IOMMU API's
back, I would want attach/detach to be as lightweight as possible.

Was there actually a good reason for having the device link to the
iommu rather than the other way around? How much would people hate it
if I just ignore the generic bindings and use something that works for
me instead. I mean, it isn't exactly like there is going to be .dts
re-use across different SoC's.. and at least with current IOMMU API
some sort of of_get_named_iommu() API doesn't really make sense.

Maybe I'm missing something, I kinda jumped into the discussion late.

BR,
-R


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