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

[Olav Haugan]

On 7/9/2014 3:54 AM, Will Deacon wrote:
> On Wed, Jul 09, 2014 at 02:07:38AM +0100, Olav Haugan wrote:
>> On 6/30/2014 2:52 AM, Will Deacon wrote:
>>> On Fri, Jun 27, 2014 at 11:23:27PM +0100, Olav Haugan wrote:
>>>> 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?
>>>
>>> You would aim to use the smallest number of SMRs per master possible.
>>> You could probably use:
>>>
>>>   Master 1: SMR[0].id == 0x20, SMR[0].mask = 0x07
>>>             SMR[1].id == 0x28, SMR[1].mask = 0x00
>>>
>>>   Master 2: SMR[2].id == 0x29, SMR[2].mask = 0x00
>>
>> So how does an algorithm figure this out in both my examples? The
>> algorithm would have to know about both (all) bus masters and their
>> stream IDs for a specific SMMU. If the algorithm operates on the set of
>> stream IDs for one bus master at a time the algorithm has no way of
>> knowing which bits can be ignored since it doesn't know the value of the
>> other stream IDs for the other bus masters and thus could potentially
>> create a mask that could cause a stream ID to match in two different
>> entries.
> 
> Complete knowledge of the system topology (i.e. all bus masters) is a
> requirement for being able to configure the SMMU correctly if you want to
> guarantee that you don't have SMR aliasing issues.

So you agree that an algorithm needs to know about all the bus
masters/stream IDs for a specific IOMMU before it can figure out the
StreamID masks and how many SMRs can be allocated to a specific bus
master? Andreas's algorithm does not know about the other bus
masters/stream IDs. It operates on one bus master at a time.

>>>> I am not familiar with Andreas's proposal. Do you have a link?
>>>
>>>   http://marc.info/?l=linux-arm-kernel&m=139110598005846&w=2
>>
>> Unless I am mistaken the algorithm works on one bus master at a time. I
>> don't think that will work.
> 
> IIRC, it works for densely packed SIDs on the master, so it tries to build
> up power-of-2 sized groups for that master then mops up the rest with
> individual entries.

I ran the algorithm through a few trivial cases:

1)
Stream IDs: 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28
Number of SMRs: 9

In this case the algorithm decided to set mask to 0 for all entries
using up 8 of the SMRs.

2) Same Stream IDs but only 2 SMRs.
The algorithm gave an error saying I did not have enough SMRs.

Thanks,

Olav

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