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

[Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>]

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. Or,
perhaps we should introduce a new compatible value that only triggers
child instantiation but not resource setup, or something like that?