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

Will Deacon
2014-06-30T09:52:20Z

Hi Olav,

On Fri, Jun 27, 2014 at 11:23:27PM +0100, Olav Haugan wrote:
> On 6/25/2014 2:18 AM, Will Deacon wrote:
> > Why can't it be dynamically detected? Whilst the StreamIDs are fixed in
> > hardware (from the SMMU architecture perspective), the SMRs are completely
> > programmable. Why doesn't something like Andreas's proposal work for you?
> > The idea there was to find the constant bits among the StreamIDs for a
> > master and create the mask accordingly.
> >
> 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

> I am not familiar with Andreas's proposal. Do you have a link?

http://marc.info/?l=linux-arm-kernel&m=139110598005846&w=2

Note that since Calxeda went under, Andreas is unfortunately no longer
working on this.

Will