Start adding checks for reg property and unit name

[Scott Wood]

On Mon, 30 Aug 2010 14:03:05 +1000
David Gibson <david at gibson.dropbear.id.au> wrote:

> In device trees, the unit name portion of any node (the part after the
> "@") is supposed to be derived from the value of the node's 'reg'
> property.  However, this is not enforced by the structure of a dtb
> file, nor is it checked by dtc.  Checking is non-trivial, because the
> manned in which the 'reg' property is encoded into a unit address can
> vary from bus to bus.
> 
> This patch starts adding the infrastructure for making such checks to
> dtc.  At present, it will only check the unit addresses on immediate
> children of the root bus (which is assumed to have a unit addresses
> encoded in plain hex) and that any other node has a unit address if
> and only if it has a 'reg' property.  However, it should be relatively
> straightforward to add more detailed checking for other sorts of
> busses from this point.

Is there any way for a user to disable this check?

First, ePAPR recommends, but does not insist on this for unit addresses,
except to the extent that a particular bus binding requires it:

> The unit-address component of the name is specific to the bus type on which the node sits. It consists
> of one or more ASCII characters from the set of characters in Table 2-1. The fundamental requirement
> is that at any level of the device tree the unit-address be unique in order to differentiate nodes with the
> same name at the same level in the tree. The binding for a particular bus may specify additional, more
> specific requirements for the format of a unit-address.
> The unit-address should match the first address specified in the reg property of the node. If the node
> has no reg property, the unit-address may be omitted if the node name alone differentiates the node
> from other nodes at the same level in the tree.

That was probably a mistake that should be corrected in the next ePAPR
revision (keeping bindings implementable on true OF is generally a good
thing), but until then it would be nice if there were at least some
mode whereby dtc could accept any valid ePAPR input.

Second, dtc is sometimes used for things that aren't semantically
device trees, and semantic checks that can't be turned off can
interfere with that usage.  We use it to compile a configuration tree
for our hypervisor, for example, and it's also used for a U-Boot image
format (with incbin).  Both of those currently use unit address syntax
without reg, and while it may a bad habit, it's not clearly wrong since
it's a different semantic domain from OF.

I guess I should write up a patch that allows individual checks to be
enabled/disabled from the command line...

Eventually a way to explicitly support alternate semantic domains, with
their own set of checks, would be nice too.

-Scott