From: David Gibson <david@gibson.dropbear.id.au>
To: Timur Tabi <timur@freescale.com>
Cc: linuxppc-dev@ozlabs.org, Arnd Bergmann <arnd@arndb.de>
Subject: Re: Could the DTS experts look at this?
Date: Wed, 13 Feb 2008 10:35:48 +1100 [thread overview]
Message-ID: <20080212233548.GE21230@localhost.localdomain> (raw)
In-Reply-To: <47B1F763.9000404@freescale.com>
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
next prev parent reply other threads:[~2008-02-12 23:35 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-02-08 23:30 Could the DTS experts look at this? Sean MacLennan
2008-02-10 5:47 ` Arnd Bergmann
2008-02-10 6:05 ` Sean MacLennan
2008-02-11 17:57 ` Timur Tabi
2008-02-11 23:54 ` David Gibson
2008-02-11 23:56 ` David Gibson
2008-02-12 0:21 ` Arnd Bergmann
2008-02-12 0:36 ` David Gibson
2008-02-12 18:51 ` Scott Wood
2008-02-12 23:17 ` David Gibson
2008-02-12 23:41 ` Timur Tabi
2008-02-12 23:50 ` David Gibson
2008-02-12 15:44 ` Timur Tabi
2008-02-12 18:58 ` Grant Likely
2008-02-12 19:08 ` Timur Tabi
2008-02-12 19:34 ` Grant Likely
2008-02-12 19:45 ` Timur Tabi
2008-02-12 20:43 ` Grant Likely
2008-02-12 23:35 ` David Gibson [this message]
2008-02-12 23:50 ` Timur Tabi
2008-02-13 0:10 ` Grant Likely
2008-02-12 23:26 ` David Gibson
2008-02-12 23:47 ` Timur Tabi
2008-02-13 0:08 ` David Gibson
2008-02-13 0:15 ` Grant Likely
2008-02-12 23:21 ` David Gibson
2008-02-11 0:14 ` David Gibson
2008-02-11 2:40 ` Sean MacLennan
2008-02-11 3:11 ` David Gibson
2008-02-11 3:49 ` Sean MacLennan
2008-02-11 23:59 ` David Gibson
2008-02-12 1:07 ` Sean MacLennan
2008-02-12 0:20 ` David Gibson
2008-02-12 0:41 ` Sean MacLennan
2008-02-12 0:48 ` David Gibson
2008-02-12 18:52 ` Scott Wood
2008-02-12 19:03 ` Grant Likely
2008-02-12 19:10 ` Scott Wood
2008-02-17 10:22 ` David Woodhouse
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