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

[Mitch Bradley <wmb-D5eQfiDGL7eakBO8gow8eQ@public.gmane.org>]

On 6/28/2012 7:50 AM, 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. Or,
> perhaps we should introduce a new compatible value that only triggers
> child instantiation but not resource setup, or something like that?

ISTM that if you want different semantics, a different compatible value 
is the best approach.  My mental model is that "simple-bus" has 
memory-mapped children. Perhaps something like "enumerated-bus" for 
non-memory-mapped ?

>