[Mesa-dev] [PATCH v3 4/4] drm/doc/rfc: i915 DG1 uAPI

[Daniel Vetter]

On Wed, Apr 21, 2021 at 8:28 PM Tvrtko Ursulin
<tvrtko.ursulin at linux.intel.com> wrote:
> On 21/04/2021 18:17, Jason Ekstrand wrote:
> > On Wed, Apr 21, 2021 at 9:25 AM Tvrtko Ursulin
> > <tvrtko.ursulin at linux.intel.com> wrote:
> >> On 21/04/2021 14:54, Jason Ekstrand wrote:
> >>> On Wed, Apr 21, 2021 at 3:22 AM Tvrtko Ursulin
> >>> <tvrtko.ursulin at linux.intel.com> wrote:
> >>>> On 20/04/2021 18:00, Jason Ekstrand wrote:
> >>>> I am not claiming to know memory region query will end up the same, and
> >>>> I definitely agree we cannot guess the future. I am just saying rsvd
> >>>> fields are inconsequential really in terms of maintenance burden and
> >>>> have been proven useful in the past. So I disagree with the drive to
> >>>> kick them all out.
> >>>
> >>> Sure, it doesn't cost anything to have extra zeros in the struct.
> >>> However, if/when the API grows using rsvd fields, we end up with "if
> >>> CAP_FOO is set, rsvd[5] means blah" which makes for a horribly
> >>> confusing API.  As a userspace person who has to remember how to use
> >>> this stuff, I'd rather make another call or chain in a struct than try
> >>> to remember and/or figure out what all 8 rsvd fields mean.
> >>
> >> Well it's not called rsvd in the uapi which is aware of the new field
> >> but has a new name.
> >
> > Are we allowed to do that?  This is a genuine question.  When I've
> > tried in the past (cliprects), I was told we couldn't rename it even
> > though literally no one had used it in code for years.
>
> Well we did the union for pad_to_size so I thought we are allowed that
> trick at least. From my experience backward source level compatibility
> is not always there even with things like glibc. Despite that, are we
> generally required to stay backward source compatible I will not claim
> either way.

I think the anonymous union with exactly same sized field is ok. We
also try hard to be source compatible, but we have screwed up in the
past and shrugged it off. The one example that comes to mind is
extended structures at the bottom with new field, which the kernel
automatically zero-extends for old userspace. But when you recompile,
your new-old userspace might no longer clear the new fields because
the ioctl code didn't start out by memset()ing the entire struct.

But by&large we managed to not botch things up on source compat, but
it's definitely a lot tricker than ABI compat.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch