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

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Message-ID: <CAMpxmJWZpMFbrMBkLiR9q7chdamVnjw0geDf-pgKrz=AWD8mNg@mail.gmail.com>
Date:   Mon, 3 Aug 2020 22:02:50 +0200
From:   Bartosz Golaszewski <bgolaszewski@...libre.com>
To:     Kent Gibson <warthog618@...il.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 Sun, Aug 2, 2020 at 5:32 AM Kent Gibson <warthog618@...il.com> wrote:
>
> On Fri, Jul 31, 2020 at 06:05:10PM +0200, Bartosz Golaszewski wrote:
> > On Sun, Jul 26, 2020 at 3:12 AM Kent Gibson <warthog618@...il.com> wrote:

[snip!]

> > > >
> > > > > +static u64 gpioline_config_flags(struct gpioline_config *lc, int line_idx)
> > > > > +{
> > > > > +       int i;
> > > > > +
> > > > > +       for (i = lc->num_attrs - 1; i >= 0; i--) {
> > > >
> > > > Much better to read is
> > > >
> > > > unsigned int i = lc->num_attrs;
> > > >
> > > > while (i--) {
> > > >  ...
> > > > }
> > > >
> > >
> > > Really? I find that the post-decrement in the while makes determining the
> > > bounds of the loop more confusing.
> > >
> >
> > Agreed, Andy: this is too much nit-picking. :)
> >
>
> I was actually hoping for some feedback on the direction of that loop,
> as it relates to the handling of multiple instances of the same
> attribute associated with a given line.
>
> The reverse loop here implements a last in wins policy, but I'm now
> thinking the kernel should be encouraging userspace to only associate a
> given attribute with a line once, and that a first in wins would help do
> that - as additional associations would be ignored.
>
> Alternatively, the kernel should enforce that an attribute can only be
> associated once, but that would require adding more request validation.
>

I guess this would result in a lot of churn to do validation which is
largely unnecessary? To me the first in wins sounds more consistent.

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.

[snip]

Bartosz