On Tue, Feb 12, 2008 at 01:45:39PM -0600, Timur Tabi wrote:
> Grant Likely wrote:
[snip]
> > That's not a dtb version issue. That is a dtb content issue.
>
> Technically, that's true, but ...
>
> > It does
> > not warrant changing the dtb version number.
>
> Then how do you solve the problem of passing a device tree to a boot
> loader that does not know how to parse it properly? A device tree
> with these additional nodes *must* be parsed by a boot loader that
> is aware of them.
Correct. Just as you must give a dtb with the information to the
correct board to a bootloader or things won't work. Changing this is
not within the reasonable scope of what dtbs will do.
> Otherwise, it will pass the device tree as-is to
> the kernel without warning. This is a bad thing, and steps should
> be taken to prevent that. If you can think of a way to make this
> happen without changing the version number, I'd love to hear. All
> I'm hearing from you now is denial that this is a problem.
>
> >>> We've already got that issue. If you pass the device tree for the
> >>> wrong board it will still validate correctly, but the board is not
> >>> going to boot.
> >> There's nothing stopping U-Boot today from scanning the device tree and
> >> making
> >> sure the SOC's compatible node is correct. That's not currently done, but
> >> it
> >> could be.
> >
> > Fair enough, and it is also reasonable for the boot loader to look for
> > a specific property name to decide how to massage the data structure.
> > This alone does not require a dtb version change.
>
> Current versions of U-Boot do not know how to do this. So again,
> I'm asking you: how do you solve the problem of passing a device
> tree with additional nodes to a boot loader that does not know how
> to parse them properly? How do you prevent that old U-Boot from
> ignoring those nodes?
You don't. If your agent takes a dtb, dtb layout and agent must
match.
> > I'm not missing the point because I disagree entirely with the
> > addition of conditional expressions to the device tree. Instead, I
> > think properties can be added to the device tree that a bootloader can
> > look for and decide to apply conditions against them which means the
> > conditions are encoded in the boot loader, not the device tree. (the
> > device tree simply contains data which supports the boot loaders
> > decision; a rather different thing).
>
> Then why bother passing a DTB to the boot loader at all? Why not
> just have the boot loader create the device tree from scratch?
That's a perfectly acceptable option - and it's what I'd expect if a
real OF decided to add support for flattened device trees (which might
happen with ePAPR). libfdt's serial-write functions are designed for
exactly this use case.
In fact, in one way of looking at it that's always what happens: the
dtb format is defined for passing hardware information from the
bootloader to the kernel; nothing else. Passing a dtb *into* the
bootloader is just a bootloader implementation convenience, because
the possible variations on an output tree are small, so it's useful to
have a skeleton tree built-in. But in order for the bootloader to
process those variations correctly, the skeleton *must* be in the
right format. dtb input to a bootloader must match the bootloaders
expectations. This has always been true, and will continue to be
true.
--
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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