Re: [PATCH v2 05/18] gpiolib: cdev: support GPIO_GET_LINE_IOCTL and GPIOLINE_GET_VALUES_IOCTL

[Kent Gibson <warthog618@xxxxxxxxx>]

On Tue, Aug 04, 2020 at 07:47:43PM +0200, Bartosz Golaszewski wrote:
> On Tue, Aug 4, 2020 at 1:01 AM Kent Gibson <warthog618@xxxxxxxxx> wrote:
> >
> 
> [snip]
> 
> >
> > > Also: I just started going through the patches - nice idea with the
> > > GPIO attributes, I really like it. Although I need to give it a longer
> > > thought tomorrow - I'm wondering if we can maybe unify them and the
> > > flags.
> > >
> >
> > I had an earlier draft that did just that - and that is partially why
> > the loop is last in wins - I was using slot 0 as the default flags.
> > But the default flags cover a lot of use cases, including all of v1, and
> > it was simple and cheap to provide a default - and it simplified the
> > initial port of libgpiod to v2...
> >
> 
> If porting libgpiod to v2 is the only concern then I wouldn't stress
> about it. At the same time I'm wondering - is there any use-case where
> we wouldn't need the flags attribute for at least some lines? Because
> if it's always required than maybe having a default isn't that bad.
> 

The only case where flags are not required is an AS-IS request. I
have no idea what that use case is useful for, but it is in v1 and
therefore supported by v2 for backward compatibility.

So there is almost always a flags attribute, and I didn't want to
waste an attribute slot on it.

Supporting the default in the kernel is trivial - it is literally just
the default return in gpioline_config_flags:

+	}
+	return lc->flags;
+}

which would otherwise be 0.

Cheers,
Kent.