[RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices to mach-omap2)

[David Gibson]

On Fri, Nov 09, 2012 at 09:36:26PM -0600, Joel A Fernandes wrote:
> Hi Pantelis,
> 
> On Fri, Nov 9, 2012 at 2:13 AM, Pantelis Antoniou
> <panto at antoniou-consulting.com> wrote:
> 
> >>>> Option C: U-Boot loads both the base and overlay FDT files, merges them,
> >>>>       and passes the resolved tree to the kernel.
> >>>>
> >>>
> >>> Could be made to work. Only really required if Joanne wants the
> >>> cape interface to work for u-boot too. For example if the cape has some
> >>> kind of network interface that u-boot will use to boot from.
> >>>
> >>
> >> I love Grant's hashing idea a lot keeping the phandle problem for
> >> compile time and not requiring fixups.
> >>
> >> IMO it is still a cleaner approach if u-boot does the tree merging for
> >> all cases, and not the kernel.
> >>
> >> That way from a development standpoint, very little or nothing will
> >> have to be changed in kernel (except for scripts/dtc) considering we
> >> are moving forward with hashing.
> >>
> >> Also this discussed a while back but at some point is going to brought
> >> up again-  loading of dt fragment directly from EEPROM and merging at
> >> run time. If we were to implement this in kernel, we would have to add
> >> cape specific EEPROM reading code, merge the tree before it is
> >> unflattened and parse. I think doing tree merging in kernel is messy
> >> and we should do it in uboot. Ideally reading the fragment from the
> >> EEPROM for all capes and merging without worrying about version
> >> detection, Doing the merge and passing the merged blob to the kernel
> >> which (kernel) works the same way it does today.
> >
> > Not going to work, for a lot of cases. Doing it in the kernel seems to be
> > the cleaner option. There are valid use cases for doing in u-boot too.
> 
> True, if dynamic runtime stuff from userspace is what we're talking
> about, then yeah I see the important need for kernel to do the merge.
> 
> >> Alternatively to hashing, reading david gibsons paper I followed,
> >> phandle is supposed to 'uniquely' identity node. I wonder why the node
> >> name itself is not sufficient to unquiely identify. The code that does
> >> the tree walking can then just strcmp the node name while it walks the
> >> tree instead of having to find a node with a phandle number. I guess
> >> the reason is phandles are small to store as data values. Another
> >> approach can be to arrange the string block in alphabetical order
> >> (unless it already is), and store phandle as index of the node name
> >> referenced relative to the starting of the strong block. This will not
> >> affect nodes in dtb being moved around since they will still have the
> >> same index value. the problem being adding or removing nodes Changes
> >> the offsets of all other nodes in the string block as well.. Hmm.
> >>
> >
> > This is pretty radical change to the DT format, no?
> 
> Yes, true and the only way hypothetically to replace the phandle
> tree-walking mechanism is to store node paths instead of phandle which
> David pointed is too long to store, so I guess this wont work after
> all. Anyway this was an interesting exercise, thanks.

They're not too long to store, but changing to paths would break years
of existing OF practice.

-- 
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_!
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