[Ksummit-2013-discuss] [ARM ATTEND] Describing complex, non-probable system topologies

[Greg KH]

On Fri, Aug 02, 2013 at 02:03:55AM -0700, Tony Lindgren wrote:
> * Greg KH <greg at kroah.com> [130801 12:33]:
> > On Thu, Aug 01, 2013 at 07:35:31PM +0100, Will Deacon wrote:
> > > Hello,
> > > 
> > > Whilst Linux implements a bunch of different bus types (many of which
> > > are in fact virtual), devices sitting on non-probable, memory mapped
> > > buses inside SoCs typically live on either the platform_bus or the
> > > amba_bus. So far, this has worked out alright; the buses haven't needed
> > > to be visible to software and no additional software control is really
> > > required from the OS. However, as I/O coherency and hardware
> > > virtualisation capabilities start to creep into ARM-based SoCs, Linux
> > > needs to know the topology of the system on which it is running.
> > > 
> > > Naturally, this would need to be described as a device-tree binding and
> > > communicate:
> > > 
> > >   - Buses which can be configured as coherent, including which devices
> > >     on those buses can be made coherent.
> > > 
> > >   - How IOMMUs sit on the bus and interact with masters on that bus (the
> > >     current one-IOMMU-driver-per-bus may not work well for the
> > >     platform_bus).
> > 
> > I've been waiting for people to finally run into this one, and realize
> > that they shouldn't be using "platform_bus" :)
> > 
> > >   - QoS and PM constraints. This isn't really in my area, but we do have
> > >     buses that have these features and expect software to control them.
> > > 
> > >   - The system topology and linkages between buses and devices.
> > 
> > The driver core handles this really well, you just have to create new
> > busses, and don't rely on the "catch-all" platform_bus.
> 
> Hmm do you have some example of a device driver that is generic and
> is supported on platform_bus and some other bus?

Take a look at drivers/usb/host/ohci* for one example that I know of,
there are others all through the kernel as well.

greg k-h