Re: Could the DTS experts look at this?

[snip]
On Tue, Feb 12, 2008 at 01:08:24PM -0600, Timur Tabi wrote:

Grant Likely wrote:

quoted

On Feb 12, 2008 8:44 AM, Timur Tabi [off-list ref] wrote:

quoted

Arnd Bergmann wrote:

 >  I think it

quoted

is a slippery slope to try and encode conditionals into the structure;

Perhaps, which is why I said it should be simple conditionals - two
values and a comparison.

I can pretty much guarantee you that someone will find that
insufficient and want to expand the conditional representation.  This
way madness lies.

quoted

but it is entirely reasonable to encode *data* into the dt that helps
make those conditional decisions.

That's okay too, except that if we just add additional nodes that
describe conditions, then we need to make sure that whatever parses
that DTB must also parse those additional nodes.  The only way to do
that is create a new version number, like 18, which is marked as
being incompatible with the current version (17).  Otherwise, a
person could pass that DTB to an old version of U-Boot, and U-boot
will just pass it on to the kernel as-is.

No.  As Grant says, that's not what the version number is for.  It
represents the version of the encoding, not the content.  If you must
version the content (which you should try really hard to avoid) the
correct way is to add versioning properties to the root node.

quoted

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.

 >  The device tree must match what the bootloader

quoted

expects.  Changing the version number will do nothing to help this.
The version number ensures that the tree is parsable.  It does not
ensure that it is correct.

I think you're missing the point.  If we add conditional expressions
to the device tree (whether attached to a node or as part of a
separate node or whatever), we must also add a mechanism to ensure
that these conditions are parsed by the boot loader.  As far as I
know, the only mechanism that can do this is the version identifier.

No. a) the version identifier is not a mechanism for doing that and b)
the conditional mechanism is inherently agent-specific, therefore in
any case you *must* match an input incomplete tree (by which I mean
*anything* that requires processing before it's ready for consumption
by the kernel) to the specific bootloader or agent that will process
it and pass to the kernel.

-- 
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				| _way_ _around_!
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