[PATCH 2/2] arm-smmu: Add possibillity to mask streamIDs

Robin Murphy robin.murphy at arm.com
Wed Jun 8 04:11:30 PDT 2016

On 08/06/16 08:47, Thommy Jakobsson wrote:
> On 6 June 2016 at 14:47, Robin Murphy <robin.murphy at arm.com> wrote:
>> That rationale doesn't hold, since "necessary" is clearly untrue - devices
>> with multiple IDs work just fine even with the current behaviour of the
>> driver provided there are sufficient SMRs. The third option of generating a
>> mask dynamically from a set of IDs is also perfectly possible (which reminds
>> me I must dig up the code I prototyped a while back...), although an
>> explicitly specified mask can still be beneficial for optimal allocation in
>> certain cases (by being able to express the non-existence of IDs outside the
>> set).
> Thanks for your time Robin. I was probably unclear in my comment. You're
> right and devices with multiple IDs do work already today. Its more the second
> part that you talk about that limits me, "provided there are
> sufficient SMRs". In
> the ZynqMPSoC, 6 of the AXI ID bits are copied into the stream id for FPGA
> IPs, so I have 2^6 IDs per master and there is 6 masters. Besides the FPGA
> there is also ethernet and dma masters (also gpu and sata, but I don't use
> those). The SMMU implementation only has 48 register groups to play with, so
> without mask I'm screwed =)

Indeed, I wasn't implying you don't need it, merely nitpicking that the 
justification was worded badly.

>> But this directly contradicts the example in the binding doc? (Which
>> incidentally wasn't sent to the devicetree mailing list.)
> Not sure that I follow, what contradicts? My intention was that for masters that
> explicitly is configured with a mask, all of its IDs should have a mask. Without
> specifying a mask it should be same behavior as before. The doc example
> shows a master, dma2, with one ID and one mask.

Oh, wait, it's only comparing the number of arguments to each phandle, 
not the whole property length, my mistake - sorry!

>> What if the DT is wrong and the SMMU doesn't support stream matching at all,
>> or the mask uses unimplemented SMR bits? Bogus stream IDs or stream match
>> conflicts are annoyingly subtle to debug.
> Point taken.
>> Either way, if you insist on wanting to build on the legacy binding that
>> doesn't work for PCI, can't integrate with DMA mapping, and everyone else
>> would rather get away from in favour of the generic IOMMU bindings[1],
> Generic IOMMU bindings would be great, but I'm not sure that it solves
> my problem
> with too many IDs vs SMRs. For a smaller

My point was that the generic bindings are also trivially easy to extend 
arbitrarily by simply increasing "#iommu-cells". In that patch series 
I'm very deliberately laying the groundwork for supporting an SMR mask 
in the second cell ;)

>> there's still no need for yet another binding to support. You can easily
>> extend "mmu-masters" to encode an SMR mask in the upper halfword of a cell
>> without ambiguity and without breaking compatibility with existing DTs.
> How do you mean breaking compatibility with existing DTs? It was my
> intention not
> to do so but instead adding an optional mask for cases when actually needed.

I didn't mean that your proposal would break anything - it clearly won't 
- but that if we were to redefine "mmu-masters" such that for for 
stream-matching SMMUs each cell represents an SMR to match rather than 
just an ID, it wouldn't actually make any difference to the behaviour on 
existing systems.

>  From a user perspective I would personally consider it a bit easier to
> understand
> with separate bindings for mask and ID, but to be honest I haven't worked that
> much with DT bindings before (which is also why I missed sending it to the DT
> mailinglist). So if you see a problem with an increasing mass of
> bindings to support,
> and consider using the upper halfword of the ID-cell a better way,
> then sure I'll buy
> that.

Either way, I'm currently in the process of reworking the SMMUv2 generic 
bindings series, having got the SMMUv3 patches out of the way, so I'll 
make sure that at least basic masking support actually makes the cut 
this time round.


> BR,
> Thommy Jakobsson

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