Subject: [RFC PATCH v3 5/7] dma-mapping: detect and configure IOMMU in of_dma_configure

From: [email-masked] (Will Deacon)
Date: Mon, 27 Oct 2014 10:51:59 +0000

On Tue, Oct 14, 2014 at 01:53:59PM +0100, Laurent Pinchart wrote:
> Hi Will,

Hi Laurent,

> On Monday 22 September 2014 18:50:27 Will Deacon wrote:
> > On Mon, Sep 22, 2014 at 10:29:10AM +0100, Thierry Reding wrote:
> > > Agreed. I wonder how useful it is to know the set of IOMMU instances
> > > that each device can master through. Wouldn't it be more useful to keep
> > > a list of master interfaces for each device? The set of IOMMU instances
> > > can trivially be derived from that.
> > 
> > I'm struggling to think how that would look. What do you mean by `master
> > interfaces' in terms of the code we have in Linux? At the end of the day,
> > the list of IOMMU instances (i.e. iommu_dma_mapping) exists because you
> > and Laurent have use-cases involving devices mastering through multiple
> > IOMMUs. If it doesn't work for you, it might be best for you to send me
> > the patch ;)
> 
> Just for the record, I've brought up the topic of masters being served by 
> multiple IOMMUs, but don't have a use case for it (yet at least). I do have 
> masters served through multiple streams with separate stream IDs, but all by 
> the same IOMMU.

Ok. I spoke to Arnd, David and Joerg at LPC and the consensus was that the
DMA-mapping API should *not* be exposed to the details of masters that
master through multiple IOMMUs. Instead, that should be abstracted by the
device API by exposing that device as a single struct device.

So, that's certainly an area that needs more work and I'll drop the limited
support I'd cooked up from this patch set in the next version.

Will