Re: [RFC PATCH] Documentation: devicetree: add description for generic bus properties

[Dave Martin <Dave.Martin-5wv7dgnIgG8@public.gmane.org>]

On Thu, Nov 28, 2013 at 10:28:45AM +0000, Will Deacon wrote:
> Hi Greg,
> 
> On Wed, Nov 27, 2013 at 11:06:50PM +0000, Greg KH wrote:
> > On Wed, Nov 27, 2013 at 05:28:06PM +0000, Dave Martin wrote:
> > > >From will.deacon-5wv7dgnIgG8@public.gmane.org Wed Nov 20 12:06:22 2013
> > > A number of discussion points remain to be resolved:
> > > 
> > >   - Use of the ranges property and describing slave vs master bus
> > >     address ranges. In the latter case, we actually want to describe our
> > >     address space with respect to the bus on which the bus masters,
> > >     rather than the parent. This could potentially be achieved by adding
> > >     properties such as dma-parent and dma-ranges (already used by PPC?)
> > > 
> > >   - Describing masters that master through multiple different buses
> > > 
> > >   - How on Earth this fits in with the Linux device model (it doesn't)
> > 
> > How does this _not_ fit into the Linux device model?  What am I missing
> > here that precludes the use of the "driver/device/bus" model we have
> > today?

The physical-sockets history of buses like PCI tends to force a simple
tree-like topology as a natural consequence.  You also end up with
closely linked topologies for power, clocks, interrupts etc., because
those all pass through the same sockets, so it's difficult to have a
different structure.

On SoC, those constraints have never existed and are not followed.  A
device's interface to the system is almost always split into multiple
connections, not covered by a single design or standard.  The problem
now is that increasing complexity means that the sometimes bizarre
topology features of SoCs are becoming less and less transparent for
software.

The device model currently seems to assume that certain things (power,
DMA and MMIO accessibility) follow the tree (which may not work for many
SoCs), and some other things (clocks, regulators, interrupts etc.) are
not incorporated at all -- making them independent, but it may make some
abstractions impossible today.

How much this matters for actual systems is hard to foresee yet.  Since
not _all_ possible insanities find their way into silicon.  The
onus should certainly be on us (i.e., the ARM/SoC community) to
demonstrate if the device model needs to change, and to find practical
ways to change it that minimise the resulting churn.

> The main problem is that we have devices which slave on one bus and master
> on another. That then complicates probing, power-management, IOMMU
> configuration, address mapping (e.g. I walk the slave buses to figure out
> where the slave registers live, but then I need a way to work out where
> exactly I master on a different bus) and dynamic coherency, amongst other
> things.
> 
> If we try to use the current infrastructure then we end up with one bus per
> device, which usually ends up being a fake bus representing both the slave
> and master buses (which is how the platform bus gets abused) and then device
> drivers having their own idea of the system topology where it's required.
> 
> This is fairly horrible and doesn't work for anything other than the trivial
> case, where one or both of the buses are `dumb' and don't require any work
> from Linux.

If we can come up with some generic bus type that is just a container for
a load of hooks that know how to deal with various aspects of each device's
interface to the system, on a per-device basis, than may be a start.

The platform bus kinda serves that role, but the trouble with that is that
it doesn't encourage any abstraction at all.  In the face of increasing
complexity, abstraction is desperately needed.

> > >  .../devicetree/bindings/arm/coherent-bus.txt       | 110 +++++++++++++++++++++
> > 
> > Why "arm"?
> > 
> > What makes it ARM specific?
> 
> This is just an RFC, so I'd be happy to put the binding somewhere more
> broad. I'm not sure how much of an issue this is outside of the SoC space,
> though.

I think that the ARM community are the ones who care the most today,
so are likely to make the most noise about it.

The binding is entirely generic in concept, so we should certainly
push for it to be non-ARM-specific.  Non-ARM SoCs will likely need
to solve this problem too at some point.

Cheers
---Dave
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