Re: [Embedded-pv-devel] [PATCH v14] This is the ABI for the two halves of a para-virtualized sound driver to communicate with each to other.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.11.16 at 19:44, <andr2000@xxxxxxxxx> wrote:
> On 11/29/2016 08:30 PM, Dario Faggioli wrote:
>> On Tue, 2016-11-29 at 19:27 +0200, Oleksandr Andrushchenko wrote:
>>> On 11/29/2016 07:05 PM, Jan Beulich wrote:
>>>> If you document it as padding, you can't easily use it later on for
>>>> some extension.
>>> Why not? I would be more careful about reserved, rather than padding.
>>> Reserved means that it might be used for something, but padding at
>>> the
>>> end of the structure (clearly?) says it was added just to align the
>>> size of
>>> this structure and most probably is not used
>>>
>> I think that's exactly the point. Padding must be zeroed and can
>> (should?) be checked to be zero.
>>
>> That means that if, say in 2 years time, we want to support a new fancy
>> feature being introduced in sound cards, and that requires adding a new
>> field in the struct, we can't use these 27 bytes, because we can't set
>> them to anything else than a bunch of 0s.
>>
>> In fact, if you use them in frontend, and happen to speak with a
>> backend that does not support the extension and enforces the padding to
>> be 0, you're doomed. :-/
>>
>> OTOH, if you say reserved, neither of the endpoints is authorized to
>> assume anything about the content of that area. Therefore:
>> 1) you can (with some care) use it for extensions
>> 2) if you do that in a frontend, even when speaking with a backend that
>> does not support the extension, it will just ignore the new content
>> (which is still just reserved space for him), and won't crash the
>> communication
>>
>> Hope this is both correct and clear. :-)
> Indeed, it does sound reasonable
> Then, should I turn all paddings in all structures into reserved?

Yes, I think so.

> Also, should I remove this:
> "All reserved and padding fields in the structures below must be 0."

Definitely not.

Jan


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