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

[Will Deacon <will.deacon@arm.com>]

On Tue, Jun 24, 2014 at 07:20:56PM +0100, Arnd Bergmann wrote:
> On Tuesday 24 June 2014 19:11:50 Will Deacon wrote:
> > On Tue, Jun 24, 2014 at 06:57:44PM +0100, Olav Haugan wrote:
> > > We do describe the masked StreamID (SID) but we need to specify the mask
> > > that the SMMU should apply to the incoming SIDs, right?
> > > 
> > > We have a bus master that emits 43 unique SIDs. However, we have only 40
> > > SMMU_SMRn registers in the SMMU. So we need to mask out some of the
> > > incoming SID bits so that the 43 SIDs can match one of 40 entries in the
> > > SMR.
> > 
> > Hmm, so you're talking about stream matching, right? That doesn't belong in
> > the device-tree. I appreciate that the current driver does a terrible job at
> > allocating the SMRs (it's bloody difficult!), but we should try to improve
> > the dynamic behaviour instead of moving configuration of the SMMU out into
> > device-tree, where it's inflexible at best.
> > 
> > There have been patches previously posted by Andreas Herrmann helping here.
> > I'd be glad to see them revived.
> 
> Note that there are areas where we have in the past decided that dynamic
> configuration is just too hard for the kernel to do and that we're better
> off putting the configuration into DT. Pinctrl and clocks are at least
> partially in that category.
> 
> It's always best if you can get the kernel to do things in the ideal
> way where that is possible, but getting there may be just not worth it.
> 
> I have no idea where it should be for SMMU, but it's something to consider:
> if you can take reasonable shortcuts by reading parts of the configuration
> from DT, you may just as well do that.

I treat this in the same manner as the topology bindings we discussed
previously; we should do a best-effort to configure things dynamically and
solve corner-cases and quirks as special cases.

Will