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 8:21 AM, Mitch Bradley wrote:
> 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 ?


I just looked at the code in platform.c that calls 
of_translate_address() (in of_device_make_bus_id()).  It appears that 
the reason for translating the address is to create a unique device name 
via:

     dev_set_name(dev, "%llx.%s", addr, node->name);

Failure of_translate_address() is not treated as an error, rather the 
code falls back to naming the device with an incrementing number:

     dev_set_name(dev, "%s.%d", node->name, magic - 1);

Perhaps the solution is add an additional step to the heuristic.  If 
of_translate_address() fails, try to use the untranslated reg property 
value to make the name unique, generating a name like:

     name.N,M

where N,M is a list of #address-cells numbers from the first entry of 
the reg property value.

That would be better than the existing heuristic (which includes the 
phrase "(and pray ...)" in its commentary), and fully consistent with 
device tree dogma.

One could still add a new compatible value to distinguish that from a 
"simple-bus", as platform.c has a match table that includes simple-bus 
and others.  The heuristic for making the unique name is not contingent 
upon a specific bus name.


>
>>
>
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