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

[Russell King - ARM Linux]

On Fri, Aug 02, 2013 at 04:36:31AM +0800, Greg KH wrote:
> On Thu, Aug 01, 2013 at 09:18:23PM +0100, Russell King - ARM Linux wrote:
> > On Fri, Aug 02, 2013 at 04:15:39AM +0800, Greg KH wrote:
> > > I'm not saying move away from DT at all, if it can be used to describe
> > > stuff like this, wonderful.  Just please don't use platform_bus anymore
> > > than you have to.
> > 
> > As far as that sentiment goes, it would have been nice if that was made
> > more vocally ten years ago, because at that time I was the one trying to
> > encourage people to think about creating appropriate bus types, and what
> > I was being told was that no, bus types are something which are deprecated
> > and platform bus is what should be used.
> 
> Was that me that said that?

I don't remember...

> I don't recall it at all, and if I did, I
> was flat out wrong.  I've always said that platform_bus is a hack, and
> should only be used as a "last resort".  Others have grabbed onto it as
> the "only" way to do devices for embedded things because that is what
> they were used to.

Well, I have three bus types to my name: the amba bus type, the ecard
bus type, and the sa1111 companion chip bus type.

I've been under pressure a number of times to convert the amba_bus
implementation to be a platform_bus, and of course I've refused to do
that because I don't see that platform buses provide what's required
there.  Not only that, but people keep using platform devices/driver
when they create a new driver which should be using the amba bus stuff.

Unfortunately, the message that platform devices should be used is soo
ingrained today that it's going to be really difficult to fight it
without basically refusing everything that comes along.

We also have the new problem (as of the adoption of DT on ARM) that it's
also embedded into the DT representation now, because the platform bus
is the "simple-bus" type in DT, and that's where everyone in ARM land
is placing their on-SoC devices.  That happened because DT already had
a way to create platform devices, and as they were already being used
on ARM, it gave a way to have DT create the platform devices without
any additional effort.

Yes, we can change the kernel code, but that now means that rather than
just changing the entirety of ARM, there's also the impact on PowerPC
to think about too with such a change.

Or we have to rip up our existing DT files and start with a different
approach...