From mboxrd@z Thu Jan  1 00:00:00 1970
From: benh@kernel.crashing.org (Benjamin Herrenschmidt)
Date: Fri, 29 Apr 2011 21:47:21 +1000
Subject: [RFC] ARM DMA mapping TODO, v1
In-Reply-To: <201104291326.25634.arnd@arndb.de>
References: <201104212129.17013.arnd@arndb.de>
	<20110428093039.GU17290@n2100.arm.linux.org.uk>
	<1304024836.2513.198.camel@pasglop> <201104291326.25634.arnd@arndb.de>
Message-ID: <1304077641.2513.242.camel@pasglop>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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.