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

Will Deacon
2014-07-09T10:54:41Z

On Wed, Jul 09, 2014 at 02:07:38AM +0100, Olav Haugan wrote:
> On 6/30/2014 2:52 AM, Will Deacon wrote:
> > On Fri, Jun 27, 2014 at 11:23:27PM +0100, Olav Haugan wrote:
> >> Lets say I have an IOMMU with 2 masters and 2 SMRn slots with the
> >> following stream IDs coming from the masters:
> >>
> >> Master 1: 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28
> >> Master 2: 0x30
> >>
> >> To make this work I would program SMR[0] with StreamID 0x20 and mask 0xF
> >> to ignore lower 4 bits. SMR[1] would just be StreamID 0x30 with mask 0x0.
> >>
> >> However, I could also have an IOMMU with 2 masters and 9 SMRn slots with
> >> the following stream IDs:
> >>
> >> Master 1: 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28
> >> Master 2: 0x29
> >>
> >> Here I would program all SMRn and leave the mask to be 0 for all SMRn's.
> >> So how do I detect when to apply a mask or not?
> >
> > You would aim to use the smallest number of SMRs per master possible.
> > You could probably use:
> >
> > Master 1: SMR[0].id == 0x20, SMR[0].mask = 0x07
> > SMR[1].id == 0x28, SMR[1].mask = 0x00
> >
> > Master 2: SMR[2].id == 0x29, SMR[2].mask = 0x00
>
> So how does an algorithm figure this out in both my examples? The
> algorithm would have to know about both (all) bus masters and their
> stream IDs for a specific SMMU. If the algorithm operates on the set of
> stream IDs for one bus master at a time the algorithm has no way of
> knowing which bits can be ignored since it doesn't know the value of the
> other stream IDs for the other bus masters and thus could potentially
> create a mask that could cause a stream ID to match in two different
> entries.

Complete knowledge of the system topology (i.e. all bus masters) is a
requirement for being able to configure the SMMU correctly if you want to
guarantee that you don't have SMR aliasing issues.

> >> I am not familiar with Andreas's proposal. Do you have a link?
> >
> > http://marc.info/?l=linux-arm-kernel&m=139110598005846&w=2
>
> Unless I am mistaken the algorithm works on one bus master at a time. I
> don't think that will work.

IIRC, it works for densely packed SIDs on the master, so it tries to build
up power-of-2 sized groups for that master then mops up the rest with
individual entries.

Will