Re: [PATCH 0/2] arm64 SMMUv3 PMU driver with IORT support

[Linu Cherian]

Hi Robin,

On Mon Dec 18, 2017 at 02:48:14PM +0000, Robin Murphy wrote:
> On 10/12/17 02:35, Linu Cherian wrote:
> >Hi,
> >
> >
> >On Fri Aug 04, 2017 at 03:59:12PM -0400, Neil Leeder wrote:
> >>This adds a driver for the SMMUv3 PMU into the perf framework.
> >>It includes an IORT update to support PM Counter Groups.
> >>
> >
> >In one of Cavium's upcoming SOC, SMMU PMCG implementation is such a way
> >that platform bus id (Device ID in ITS terminmology)is shared with that of SMMU.
> >This would be a matter of concern for software if the SMMU and SMMU PMCG blocks
> >are managed by two independent drivers.
> >
> >The problem arises when we want to alloc/free MSIs for these devices
> >using the APIs, platform_msi_domain_alloc/free_irqs.
> >Platform bus id being same for these two hardware blocks, they end up sharing the same
> >ITT(Interrupt Translation Table) in GIC ITS and hence alloc, free and management
> >of this shared ITT becomes a problem when they are managed by two independent
> >drivers.
> 
> What is the problem exactly? IIRC resizing a possibly-live ITT is a
> right pain in the bum to do - is it just that?

Yes exactly. Resizing ITT was the problem in sharing.

 
> >We were looking into the option of keeping the SMMU PMCG nodes as sub nodes under
> >SMMUv3 node, so that SMMUv3 driver could probe and figure out the total vectors
> >required for SMMU PMCG devices and make a common platform_msi_domain_alloc/free_irqs
> >function call for all devices that share the platform bus id.
> 
> I'm not sure how scalable that approach would be, since it's not
> entirely obvious how to handle PMCGs associated with named
> components or root complexes (rather than directly with SMMU
> instances). We certainly don't want to end up spraying similar PMCG
> DevID logic around all manner of GPU/accelerator/etc. drivers in
> future (whilst PMCGs for device TLBs will be expected to have
> distinct IDs from their host devices, they could reasonably still
> overlap with other PMCGs/SMMUs).
>

OK.
 
While trying the above approach, we also felt that the code will become 
lot messier than actually thought.

> >Would like to know your expert opinion on what would be the right approach
> >to handle this case ?
> 
> My gut feeling says the way to deal with this properly is in the ITS
> code, but I appreciate that that's a lot easier said than done :/
>

Yes Correct.
 
> Robin.

-- 
Linu cherian