Re: [RFC] ARM DMA mapping TODO, v1

[Benjamin Herrenschmidt]

On Fri, 2011-04-29 at 13:26 +0200, Arnd Bergmann wrote:
> On Thursday 28 April 2011, Benjamin Herrenschmidt wrote:
> > > > 
> > > > 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.
> > 
> > But that's not PCI right ? IE. with PCI, coherency is a property of the
> > PHB... 
> 
> That is my understanding at least, but I'd like to have a confirmation
> from the DRM folks.
> 
> I believe that the PC graphics cards that have noncoherent DMA mappings
> are all of the unified memory (integrated into the northbridge) kind,
> so they are not on the same host bridge as all regular PCI devices,
> even if they appear as a PCI device.

Hrm... beware with x86 , they love playing tricks :-) Since too many
BIOSes don't understand PCI domains, they make devices on separate
bridges look like sibling on the same segment and that sort of thing.

Cheers,
Ben.

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