Re: [PATCH v9 04/20] gpio: uapi: define uAPI v2

[Andy Shevchenko]

On Wed, Sep 23, 2020 at 1:30 PM Kent Gibson <warthog618@xxxxxxxxx> wrote:
>
> On Wed, Sep 23, 2020 at 01:04:05PM +0300, Andy Shevchenko wrote:
> > On Tue, Sep 22, 2020 at 5:34 AM Kent Gibson <warthog618@xxxxxxxxx> wrote:
> > >
>
> [snip]
>
> > > There is also some minor renaming of fields for consistency compared to
> > > their v1 counterparts, e.g. offset rather than lineoffset or line_offset,
> > > and consumer rather than consumer_label.
> > >
> > > Additionally, v1 GPIOHANDLES_MAX becomes GPIO_V2_LINES_MAX in v2 for
> > > clarity, and the gpiohandle_data __u8 array becomes a bitmap in
> > > gpio_v2_line_values.
> > >
> > > The v2 uAPI is mostly a reorganisation and extension of v1, so userspace
> > > code, particularly libgpiod, should readily port to it.
> >
> > ...
> >
> > > +struct gpio_v2_line_config {
> > > +       __aligned_u64 flags;
> > > +       __u32 num_attrs;
> >
> > > +       /* Pad to fill implicit padding and reserve space for future use. */
> > > +       __u32 padding[5];
> >
> > Probably I somehow missed the answer, but why do we need 5 here and not 1?
> >
>
> Sorry, I got tired of repeating myself, and just acked that we disagree
> on the approach here.
>
> Your suggestion to use the size for version would result in an
> explosion of ioctl signatures - every time we add a field we have to add
> a new ioctl and handle it separately in gpio_ioctl() or linereq_ioctl().

No, you just add
__u32 version; // implies sizeof() check as well.

Look for examples of existing ABIs (e.g. perf ABI).

> Instead what we do here is reserve some space for future use - that we
> can replace with fields without changing the signature.
> The padding is required to be zeroed now, and any future use will take
> a 0 to mean "leave alone".
>
> The sizes are a guestimate as to what may be needed in the future, and
> as such are almost certainly wrong - but hopefully on the high side.
> If that fails we can always fall back to your approach.

I see. So, we have no agreement on these pieces.
Linus and Bart can decide what to do, but I think either way has pros and cons.

So, guys, I am fine with everything else here, except this versioning
issue and waste of resources.

-- 
With Best Regards,
Andy Shevchenko