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

Olav Haugan
2014-06-27T22:23:33Z

On 6/25/2014 2:18 AM, Will Deacon wrote:
> On Tue, Jun 24, 2014 at 10:35:54PM +0100, Olav Haugan wrote:
>> On 6/24/2014 11:11 AM, Will Deacon wrote:
>>> On Tue, Jun 24, 2014 at 06:57:44PM +0100, Olav Haugan wrote:
>>>> On 6/24/2014 2:18 AM, Will Deacon wrote:
>>>>> On Sat, Jun 21, 2014 at 12:16:25AM +0100, Olav Haugan wrote:
>>>>>> 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>;
>>>>>> };
>>>>>
>>>>> Stupid question, but why not simply describe the masked IDs? What use does
>>>>> the `raw' ID have to Linux?
>>>>
>>>> We do describe the masked StreamID (SID) but we need to specify the mask
>>>> that the SMMU should apply to the incoming SIDs, right?
>>>>
>>>> We have a bus master that emits 43 unique SIDs. However, we have only 40
>>>> SMMU_SMRn registers in the SMMU. So we need to mask out some of the
>>>> incoming SID bits so that the 43 SIDs can match one of 40 entries in the
>>>> SMR.
>>>
>>> Hmm, so you're talking about stream matching, right? That doesn't belong in
>>> the device-tree. I appreciate that the current driver does a terrible job at
>>> allocating the SMRs (it's bloody difficult!), but we should try to improve
>>> the dynamic behaviour instead of moving configuration of the SMMU out into
>>> device-tree, where it's inflexible at best.
>>
>> I am talking about SMMU_SMRn[MASK] register bits. This is not something
>> that can be dynamically detected at run-time. It is configuration at the
>> same level as the actual StreamIDs.
>
> Why can't it be dynamically detected? Whilst the StreamIDs are fixed in
> hardware (from the SMMU architecture perspective), the SMRs are completely
> programmable. Why doesn't something like Andreas's proposal work for you?
> The idea there was to find the constant bits among the StreamIDs for a
> master and create the mask accordingly.
>
Lets say I have an IOMMU with 2 masters and 2 SMRn slots with the
following stream IDs coming from the masters:

Master 1: 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28
Master 2: 0x30

To make this work I would program SMR[0] with StreamID 0x20 and mask 0xF
to ignore lower 4 bits. SMR[1] would just be StreamID 0x30 with mask 0x0.

However, I could also have an IOMMU with 2 masters and 9 SMRn slots with
the following stream IDs:

Master 1: 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28
Master 2: 0x29

Here I would program all SMRn and leave the mask to be 0 for all SMRn's.
So how do I detect when to apply a mask or not?

I am not familiar with Andreas's proposal. Do you have a link?

Thanks,

Olav Haugan

--
The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by The Linux Foundation