Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

[Olav Haugan]

On 5/30/2014 12:06 PM, Arnd Bergmann wrote:
> On Friday 30 May 2014 08:16:05 Rob Herring wrote:
>> On Fri, May 23, 2014 at 3:33 PM, Thierry Reding
>> <thierry.reding@xxxxxxxxx> wrote:
>>> From: Thierry Reding <treding@xxxxxxxxxx>
>>> +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.
>>
>> Just thinking out loud, could you have dma-ranges in the iommu node
>> for the case when the iommu is enabled rather than putting the DMA
>> window information into the iommus property?
>>
>> This would probably mean that you need both #iommu-cells and #address-cells.
> 
> The reason for doing like this was that you may need a different window
> for each device, while there can only be one dma-ranges property in 
> an iommu node.
> 
>>> +
>>> +Optional properties:
>>> +--------------------
>>> +- iommu-names: A list of names identifying each entry in the "iommus"
>>> +  property.
>>
>> Do we really need a name here? I would not expect that you have
>> clearly documented names here from the datasheet like you would for
>> interrupts or clocks, so you'd just be making up names. Sorry, but I'm
>> not a fan of names properties in general.
> 
> Good point, this was really overdesign by modeling it after other
> subsystems that can have a use for names.
>  
>>> +Multiple-master IOMMU:
>>> +----------------------
>>> +
>>> +       iommu {
>>> +               /* the specifier represents the ID of the master */
>>> +               #address-cells = <1>;
>>> +               #size-cells = <0>;
>>> +       };
>>> +
>>> +       master {
>>> +               /* device has master ID 42 in the IOMMU */
>>> +               iommus = <&/iommu 42>;
>>> +       };
>>
>> Presumably the ID would be the streamID on ARM's SMMU. How would a
>> master with 8 streamIDs be described? This is what Calxeda midway has
>> for SATA and I would expect that to be somewhat common. Either you
>> need some ID masking or you'll have lots of duplication when you have
>> windows.
> 
> I don't understand the problem. If you have stream IDs 0 through 7,
> you would have
> 
> 	master@a {
> 		...
> 		iommus = <&smmu 0>;
> 	};
> 
> 	master@b {
> 		...
> 		iommus = <&smmu 1;
> 	};
> 
> 	...
> 
> 	master@12 {
> 		...
> 		iommus = <&smmu 7;
> 	};
> 
> and you don't need a window at all. Why would you need a mask of
> some sort?

We have multiple-master SMMUs and each master emits a variable number of
StreamIDs. However, we have to apply a mask (the ARM SMMU spec allows
for this) to the StreamIDs due to limited number of StreamID 2 Context
Bank entries in the SMMU. If my understanding is correct we would
represent this in the DT like this:

	iommu {
		#address-cells = <2>;
		#size-cells = <0>;
	};

	master@a {
		...
		iommus = <&iommu StreamID0 MASK0>,
			 <&iommu StreamID1 MASK1>,
			 <&iommu StreamID2 MASK2>;
	};

	master@b {
		...
		iommus = <&iommu StreamID3 MASK3>,
			 <&iommu StreamID4 MASK4>;
	};


Thanks,

Olav Haugan

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