Re: [PATCH 5/6] dma-mapping: support fsl-mc bus

[Robin Murphy <robin.murphy@arm.com>]

On 05/03/18 18:39, Christoph Hellwig wrote:
> On Mon, Mar 05, 2018 at 03:48:32PM +0000, Robin Murphy wrote:
>> Unfortunately for us, fsl-mc is conceptually rather like PCI in that it's
>> software-discoverable and the only thing described in DT is the bus "host",
>> thus we need the same sort of thing as for PCI to map from the child
>> devices back to the bus root in order to find the appropriate firmware
>> node. Worse than PCI, though, we wouldn't even have the option of
>> describing child devices statically in firmware at all, since it's actually
>> one of these runtime-configurable "build your own network accelerator"
>> hardware pools where userspace gets to create and destroy "devices" as it
>> likes.
> 
> I really hate the PCI special case just as much.  Maybe we just
> need a dma_configure method on the bus, and move PCI as well as fsl-mc
> to it.

Hmm, on reflection, 100% ack to that idea. It would neatly supersede 
bus->force_dma *and* mean that we don't have to effectively pull pci.h 
into everything, which I've never liked. In hindsight dma_configure() 
does feel like it's grown into this odd choke point where we munge 
everything in just for it to awkwardly unpick things again.

Robin.