linux-kernel - Re: [PATCH v2 05/18] gpiolib: cdev: support GPIO_GET_LINE_IOCTL and GPIOLINE_GET_VALUES

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Date:   Wed, 5 Aug 2020 11:06:42 +0800
From:   Kent Gibson <warthog618@...il.com>
To:     Bartosz Golaszewski <bgolaszewski@...libre.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH v2 05/18] gpiolib: cdev: support GPIO_GET_LINE_IOCTL and
 GPIOLINE_GET_VALUES_IOCTL

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@...il.com> 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.