Re: [PATCH 3/5] iommu/arm-smmu: Introduce stream ID masking

[Rob Herring <robherring2-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 10/08/2013 11:59 AM, Will Deacon wrote:
> On Tue, Oct 08, 2013 at 05:40:21PM +0100, Andreas Herrmann wrote:
>> On Tue, Oct 08, 2013 at 05:20:08PM +0200, Andreas Herrmann wrote:
>> To be more specific: For SATA I'd need to specify 10 StreamIds. This
>> would
>>
>> (1) exceed MAX_MASTER_STREAMIDS (currently it's 8)
>>
>>    (Can easily be fixed by adapting a macro.)
>>
>> (2) exceed number of available SMR groups to map the IDs to a context.
>>
>>    This can be solved by caclulating an appropriate mask for the
>>    mapping (but with a non-power-of-two number of StreamIds that's
>>    already non-trivial -- for the trivial case I have some code to do
>>    this).
>>
>> Both problems are avoided by introducing this patch -- use
>> smr_mask_bits to map all StreamIDs to the same context and be done
>> with it. (for the "single-master-SMMU" case)
> 
> The problem is, this information *really* doesn't belong in the device tree,
> but I think computing the general case dynamically is incredibly difficult
> too (and requires *complete* topological information in the device-tree, so
> you don't accidentally pull in other devices).

Couldn't this information be implied from the DT when you have no
streamID and only a single mmu-master?

Rob

>> PS: I think (2) needs to be addressed sooner or later. We should use
>>     only as many SMR groups as really required -- ie. use masking of
>>     StreamIds if possible. If more than one StreamID is given for a
>>     master it might be possible to calculate a mask for a
>>     (power-of-two) number of adjacent StreamIds and then use only one
>>     SMR group to map these IDs to a context. (But I think that should
>>     only be done if multiple masters are attached to an SMMU.)
> 
> I spent a few weeks looking at doing minimal SMR grouping whilst writing the
> driver and ended up convincing myself that it's an NP-complete problem (I
> tried a reduction involving Hamiltonian Cycles). Of course, if you have some
> ideas here, we can try to implement something for a constrained instance of
> the problem.
> 
> For example, a simple solution is to xor all the IDs together and check no
> other IDs fall under the resulting mask. However, this again relies on the
> DT telling us complete topological information as well as the IDs being
> organised in a particular way.
> 
> Will
>