Re: [Ksummit-2013-discuss] DT bindings as ABI [was: Do we havepeople interested in device tree janitoring / cleanup?]

[David Gibson]

On Mon, Jul 29, 2013 at 12:05:13PM -0600, Jason Gunthorpe wrote:
> On Sat, Jul 27, 2013 at 11:51:06AM -0700, Tomasz Figa wrote:
> 
> > Well, it depends on how we use the DT. There are (at least) two possible 
> > usage scenarios:
> > 
> >  a) using DT as direct replacement for board files - this means that you 
> >     are free to say that DTSes are strictly coupled with kernel version 
> >     and you are free to modify the bindings - see the analogy to board 
> >     files, where you could modify the platform data structures and could 
> >     not directly copy board file from one kernel version to another,
> > 
> >  b) using DT as an ABI - this is the original way, i.e. define stable 
> >     bindings and make sure that anu DTB built for older kernel will work,
> >     with equal or greater set of functionality on newer kernels.
> > 
> > Now I believe in this thread the point whether we should use a) or b) or a 
> > combination of both has been raised.
> 
> Right, and I think it is very important to consider that different
> systems can legitimately fall into those categories.
> 
> Clearly general purpose systems (eg servers, workstations, etc) with
> *full featured firmware* fall into category b. Linux already basically
> has stable DT for those systems - but the firmware is expected to do
> lots of work and hide all the low level details from the
> kernel.  Basically, the DT should stick to approximately ePAR and
> everything else is hidden by the firmware.

No.  With the exception of the hypervisor/virtualization extensions,
ePAPR describes (for now) an entirely passive firmware interface.
That is, once the handover to the OS has happened, there is *no*
further firmware interaction.  It is not capable of hiding any details
from the OS, except those which can be done by one-time
initialization.

In fact, a guiding principle of ePAPR's design was that except in
cases where it's *much* easier for the firmware to do things, the OS
should be expected to do it, because the OS is usually easier to fix
than the firmware.

> This is essentially how x86
> works and achieves its compatibility.
> 
> Systems that have minimalist firmware, where firmware functions are
> pushed into the kernel and the DT is now required to describe
> intricate and unique SOC specific functions are clearly very
> different, and I think it makes sense to encourage those environments
> to be case 'a'.
> 
> Basically, minimalist firmware should have a boot process design that
> *can* couple the DT and kernel, while full featured firmware should
> keep them seperate. IMHO that should be the clear message to people
> implementing this stuff.
> 
> After enough time the DT for 'a' should become stable and churn free,
> but expecting/demanding that from day 0 is not helping anyone, IMHO.
> 
> Jason

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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