Re: [PATCH v8 1/3] iommu/arm-smmu: add NVIDIA implementation for dual ARM MMU-500 usage

[Robin Murphy]

On 2020-07-01 19:18, Krishna Reddy wrote:
> > > + * When Linux kernel supports multiple SMMU devices, the SMMU
> > > device +used for + * isochornous HW devices should be added as a
> > > separate ARM MMU-500 +device + * in DT and be programmed
> > > independently for efficient TLB invalidates.
>
> > I don't understand the "When" there - the driver has always
> > supported multiple independent SMMUs, and it's not something that
> > could be configured out or otherwise disabled. Plus I really don't
> > see why you would ever want to force unrelated SMMUs to be
> > >programmed together - beyond the TLB thing mentioned it would also
> > waste precious context bank resources and might lead to weird
> > device grouping via false stream ID aliasing, with no obvious
> > upside at all.
>
> Sorry, I missed this comment. During the initial patches, when the
> iommu_ops were different between, support multiple SMMU drivers at
> the same is not possible as one of them(that gets probed last)
> overwrites the platform bus ops. On revisiting the original issue,
> This problem is no longer relevant. At this point, It makes more
> sense to just get rid of 3rd instance programming in
> arm-smmu-nvidia.c and just limit it to the SMMU instances that need
> identical programming.

Yeah, I realised later last night that this probably originated from 
forking the whole driver downstream. But even then you could have 
treated the other one as a separate nsmmu with a single instance ;)

Since it does add a bit of confusion to the code and comments, let's 
just keep things simple. I do like Jon's suggestion of actually 
enforcing that the number of "reg" regions exactly matches the number 
expected for the given compatible - I guess for now that means just 
hard-coding 2 and hoping the hardware folks don't cook up any more of 
these...

Robin.