[RFC] ARM DMA mapping TODO, v1

[Russell King - ARM Linux]

On Thu, Apr 28, 2011 at 07:41:07AM +1000, Benjamin Herrenschmidt wrote:
> 
> > As I said above, I don't think bus code can do it.  Take my example
> > above of a tulip pci device on x86 and a tulip pci device on ARM.  Both
> > use the same PCI code.
> > 
> > Maybe something in asm/pci.h - but that invites having lots of bus
> > specific header files in asm/.
> > 
> > A better solution imho would be to have an architecture callback for
> > struct device which gets registered, which can inspect the type of
> > the device, and set the flag depending on where it appears in the
> > tree.
> 
> Now -that's gross :-)
> 
> For PCI you can have the flag propagate from the PHB down, for busses
> without a bus type (platform) then whoever instanciate them (the
> platform code) can set that appropriately.

How can you do that when it changes mid-bus heirarchy?  I'm thinking
of the situation where the DRM stuff is on a child bus below the
root bus, and the root bus has DMA coherent devices on it but the DRM
stuff doesn't.

Your solution doesn't allow that - and I believe that's what Arnd is
talking about.