Re: Question about arm smmu

[Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>]

On 04/11/15 14:43, Will Deacon wrote:
> On Wed, Nov 04, 2015 at 09:44:40PM +0800, Peng Fan wrote:
>> Hi Will,
>
> Hello,
>
>> I am trying to enable SMMU(mmu500) on an platform.
>> My platform support only 32 SIDs, but it have more than 100 masters.

Do you mean the MMU-500 configuration really only has 5 bits of stream 
ID wired up (which seems a bit unlikely, especially given that in such a 
configuration you should gain extra bits from the prepended TBU number), 
or that you have a sufficient number of stream ID bits but only have 32 
stream matching groups?

>> So I need to let different masters share one SID. I read current
>> arm-smmu.c, but it needs each master has unique SID. Do you have
>> some suggestions about how to let different masters sharing one
>> SID?
>
> We can achieve that using iommu_groups, but then we need a way to
> describe those groups in the device-tree, as opposed to putting
> each device into its own group like we do at present. Robin (CC'd) had
> some work-in-progress for this, iirc.

I spent enough time on it that I reached the point of thinking there's 
no feasible DT binding that is sufficiently hardware-agnostic, and isn't 
just trying to paint a Linux implementation detail as hardware 
description - the master IDs are already there, so it might as well be 
up to the thing parsing them to work out what, if anything, it wants to 
do with duplicates.

What I have now is a simple little lookup table solution inside the 
SMMUv2 driver to automatically track groups by stream ID for platform 
devices. I've not yet tried pulling in Joerg's generic group series, but 
I think it's going to slot together really neatly with that, so I might 
try breaking it out of the big stream ID probing rework I'm currently 
half-way through.

>> On my platform, SID can be dynamically programmed. So I can program
>> DMA0 and DMA1 using one SID, saying 0x5. But I do not have a good
>> idea how to support this use case in arm-smmu.c driver.
>
> I think we've have the firmware allocating the SIDs, then describing
> the grouping to Linux in the device-tree.

Yeah, regardless of how we handle groups we need the SIDs to be in the 
DT and not change at runtime, so the programming is going to have to be 
done before the kernel even runs.

Robin.

>
> Will
>