Re: [PATCH v4 4/8] hw/misc/pca9552: Add a 'description' property for debugging purpose

[Philippe Mathieu-Daudé]

On 6/25/20 10:12 AM, Philippe Mathieu-Daudé wrote:
> On 6/25/20 8:37 AM, Markus Armbruster wrote:
>> Cédric Le Goater <clg@kaod.org> writes:
>>
>>> On 6/22/20 10:31 AM, Philippe Mathieu-Daudé wrote:
>>>> On 6/22/20 8:27 AM, Cédric Le Goater wrote:
>>>>> On 6/21/20 12:58 AM, Philippe Mathieu-Daudé wrote:
>>>>>> Add a description field to distinguish between multiple devices.
>>
>> Pardon my lack of imagination: how does this help you with debugging?
> 
> Ah, the patch subject is indeed incorrect, this should be:
> "... for *tracing* purpose" (I use tracing when debugging).
> 
> In the next patch, we use the 'description' property:
> 
> +# pca9552.c
> +pca9552_gpio_status(const char *description, const char *buf) "%s GPIOs
> 0-15 [%s]"
> 
> So when the machine has multiple PCA9552 and guest accesses both,
> we can distinct which one is used. For me having "pca1" / "pca0"
> is easier to follow than the address of the QOM object.
> 
>>
>>>>> Reviewed-by: Cédric Le Goater <clg@kaod.org>
>>>>>
>>>>> Could it be a QOM attribute ? 
>>>>
>>>> What do you call a 'QOM attribute'?
>>>> Is it what qdev properties implement?
>>>> (in this case via DEFINE_PROP_STRING).
>>>
>>> I meant a default Object property, which would apply to all Objects. 
>>
>> Good point.  Many devices have multiple component objects of the same
>> type.
>>
>>> What you did is fine, so :
>>>
>>> Reviewed-by: Cédric Le Goater <clg@kaod.org>
>>>
>>> but, may be, a well defined child name is enough for the purpose.
>>
>> object_get_canonical_path() returns a distinct path for each (component)
>> object.  The path components are the child property names.
>>
>> Properties can have descriptions: object_property_set_description().
> 
> TIL object_property_set_description :>
> 
> Ah, there is no equivalent object_property_get_description(),
> we have to use object_get_canonical_path(). Hmm, not obvious.
> 
>>
>> Sufficient?
> 
> I don't know... This seems a complex way to do something simple...
> This is already a QDEV. Having to use QOM API seems going
> backward, since we have the DEFINE_PROP_STRING() macros available
> in "hw/qdev-properties.h".
> 
> Maybe I'm not seeing the advantages clearly. I'll try later.

The canonical path is not very helpful in trace log...

The description I set matches the hardware definitions
and is easier to follow, see patch #6 (where it is set)
where the description comes from:

https://www.mail-archive.com/qemu-devel@nongnu.org/msg714658.html

  Description name taken from:
  https://github.com/open-power/witherspoon-xml/blob/master/witherspoon.xml

So in this particular case I don't find the canonical pathname
practical (from an hardware debugging perspective).

> 
> Thanks for your review,
> 
> Phil.
>