Re: #size-cells = <0> in a bus node, and kernel messages complaining about this

[David Gibson <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org>]

On Thu, Jun 28, 2012 at 11:50:02AM -0600, Stephen Warren wrote:
> On 06/27/2012 06:57 PM, Mitch Bradley wrote:
> > On 6/27/2012 11:26 AM, Stephen Warren wrote:
> >> I believe I've seen the following construct bandied about as the correct
> >> way of representing a bunch of nodes that have the same name (since they
> >> represent the same type of object) within device-tree.
> >>
> >>     regulators {
> >>         compatible = "simple-bus";
> >>         #address-cells = <1>;
> >>         #size-cells = <0>;
> >>
> >>         regulator@0 {
> >>             compatible = "regulator-fixed";
> >>             reg = <0>;
> >> ...
> >>         };
> >>
> >>         regulator@1 {
> >>             compatible = "regulator-fixed";
> >>             reg = <1>;
> >> ...
> >>         };
> >>     };
> >>
> >> However, when the kernel parses that, it issues messages such as:
> >>
> >> prom_parse: Bad cell count for /regulators/regulator@0
> >> prom_parse: Bad cell count for /regulators/regulator@1
> > 
> > The message comes from __of_translate_address(), which has the comment:
> > 
> >  * Note: We consider that crossing any level with #size-cells == 0 to mean
> >  * that translation is impossible (that is we are not dealing with a value
> >  * that can be mapped to a cpu physical address). This is not really
> > specified
> >  * that way, but this is traditionally the way IBM at least do things
> > 
> > So it seems that the problem only occurs if something tries to translate
> > the regulator's "reg" address to a CPU address.  Is that
> > possible/meaningful
> > in your case?
> 
> That's quite likely.
> 
> Note that the regulators node is compatible = "simple-bus", and I'm
> doing that so that the child regulator nodes are automatically recursed
> into, and a platform device created for each. Part of creating those
> platform devices is to convert the reg and interrupts properties to
> platform device resources, which is almost certainly what's calling
> __of_translate_address(). This is a compatibility thing on ARM; I guess
> pure OF-style drivers call something like of_get_address()/of_iomap()
> themselves in their probe() function if appropriate, rather than relying
> on calling platform_get_resource(), and hence forcing the DT parsing
> code to convert resources beforehand in all cases, even if not used.
> 
> Perhaps simple-bus could be enhanced to detect when size-cells==0 and
> know that this means the bus is really just a container/grouping of
> non-addressed objects, and hence not provide the memory resources.

That definitely doesn't make sense.  The device tree already expresses
whether a bus is mapped into the top-level address space in the
absence of ranges properties in the parent bus.

-- 
David Gibson			| I'll have my music baroque, and my code
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				| _way_ _around_!
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