Boot interface for device trees on ARM

[David Gibson]

On Tue, May 18, 2010 at 09:28:38PM -0400, Nicolas Pitre wrote:
> On Wed, 19 May 2010, David Gibson wrote:
> 
> > On Tue, May 18, 2010 at 08:24:06AM -0400, Nicolas Pitre wrote:
> > > On Tue, 18 May 2010, David Gibson wrote:
> > > > On Tue, May 18, 2010 at 01:24:43PM +0800, Jeremy Kerr wrote:
> > > > > Nicolas Pitre wrote:
> > [snip]
> > > > The only reason you'd need a subarchitecture number or equivalent is
> > > > if you need to do things differently in the very, very early asm boot
> > > > code.  We may need a minimal form of this on PowerPC if we ever
> > > > support multiple MMU families in the same kernel binary.
> > > 
> > > Exact.  For example, on ARM the machine ID is also used to figure out 
> > > the MMU mapping needed to be able to simply be able to debug the very 
> > > early assembly boot stage when there isn't even a stack available. While 
> > > this info is stored in the machine record, it is actually 
> > > subarchitecture specific and already half-digested for easy usage by 
> > > that initial MMU setup.  I just don't want to imagine what the 
> > > equivalent functionality with DT would look like.
> > 
> > Well, it wouldn't be *that* bad - you'd need a minimal asm-only tree
> > walker to find and look up the compatible property.  Quite possible
> > but, yes, fairly awkward.
> > 
> > Yeah, if you have to make really early MMU decisions based on
> > subarchitecture it probbaly makes sense to have a simple ID for
> > those.  So one thing to consider here is that we've contemplated
> > adding an "MMU family" ID to the device tree blob header for this
> > purpose on PowerPC.  If we did such a thing, it could also be used on
> > ARM for a similar purpose.  Then all the relevant machine information
> > is in one block, and it's just a simple fixed dereference to extract
> > the MMU type from the early asm.
> > 
> > I'd have to talk to Ben et al, but I think we'd be happy enough to
> > create a new dtb v18 which included that field, as well as a couple of
> > other small improvements to the header and blob internals.  ARM could
> > standardize on that as the minimum acceptable dtb version.
> 
> I still don't get it.  Why on earth would you insist on replacing the 
> already existing ID value that is _already_ passed to the kernel by all 
> ARM bootloaders with something evidently far more complex?

Um.. no.  It's not more complex at all.  We're talking about this ID
in the dt blob's header, not in the device tree itself.  As I said,
it's one dereference to get to the ID, from the DT address which is
being passed in.  i.e. *one* more instruction than having it passed in
register.

There are two advantages, both small, but contrary to your assertion
it's not significantly more complex, so they don't need to be big.
	1) It means the DT blob contains *all* the information you
need about the machine.  If you have multi-stage bootloaders, you can
copy that blob and know you've got everything.
	2) It means we could have information with a similar purpose
in the same place for both PowerPC and ARM (and maybe others in
future).

-- 
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