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.

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

On 11/30/2016 10:45 AM, Jan Beulich wrote:
> > > > 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.
ok
> > Also, should I remove this:
> > "All reserved and padding fields in the structures below must be 0."
> Definitely not.
ok
> Jan

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