linux-kernel - Re: [PATCH v9 10/20] gpiolib: cdev: support GPIO_V2_LINE_SET_CONFIG

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Date:   Thu, 24 Sep 2020 17:26:22 +0800
From:   Kent Gibson <warthog618@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v9 10/20] gpiolib: cdev: support
 GPIO_V2_LINE_SET_CONFIG_IOCTL

On Thu, Sep 24, 2020 at 11:26:49AM +0300, Andy Shevchenko wrote:
> On Thu, Sep 24, 2020 at 6:24 AM Kent Gibson <warthog618@...il.com> wrote:
> > On Wed, Sep 23, 2020 at 07:15:46PM +0300, Andy Shevchenko wrote:
> > > On Wed, Sep 23, 2020 at 7:14 PM Andy Shevchenko
> > > <andy.shevchenko@...il.com> wrote:
> > > > On Tue, Sep 22, 2020 at 5:35 AM Kent Gibson <warthog618@...il.com> wrote:
> > > > >
> > > > > Add support for GPIO_V2_LINE_SET_CONFIG_IOCTL, the uAPI v2
> > > > > line set config ioctl.
> > >
> > > > > +static long linereq_set_config_unlocked(struct linereq *lr,
> > > > > +                                       struct gpio_v2_line_config *lc)
> > > > > +{
> > > > > +       struct gpio_desc *desc;
> > > > > +       unsigned int i;
> > > > > +       u64 flags;
> > > > > +       bool polarity_change;
> > > > > +       int ret;
> > > > > +
> > > > > +       for (i = 0; i < lr->num_lines; i++) {
> > > > > +               desc = lr->lines[i].desc;
> > > > > +               flags = gpio_v2_line_config_flags(lc, i);
> > > >
> > > > > +               polarity_change =
> > > > > +                       (test_bit(FLAG_ACTIVE_LOW, &desc->flags) !=
> > > > > +                        ((flags & GPIO_V2_LINE_FLAG_ACTIVE_LOW) != 0));
> > > >
> > > > Comparison
> > >
> > > Comparison between int / long (not all archs are agreed on this) and
> > > boolean is not the best we can do.
> > >
> >
> > There is no bool to int comparision here.
> 
> test_bit() returns int or long depending on arch... Then you compare
> it to bool (which is a product of != 0).
> 

Really - I thought it returned bool.
It is a test - why would it return int or long?
Surely it is guaranteed to return 0 or 1?

> > There are two comparisons - the inner int vs int => bool and the
> > outer bool vs bool.  The "!= 0" is effectively an implicit cast to
> > bool, as is your new_polarity initialisation below.
> >
> > > What about
> > >
> > >   bool old_polarity = test_bit(FLAG_ACTIVE_LOW, &desc->flags);
> > >   bool new_polarity = flags & GPIO_V2_LINE_FLAG_ACTIVE_LOW;
> > >
> > >   old_polarity ^ new_polarity
> >
> > So using bitwise operators on bools is ok??
> 
> XOR is special. There were never bitwise/boolean XORs.
> 

We must live in different universes, cos there has been a bitwise XOR in
mine since K&R.  The logical XOR is '!='.

> > > and move this under INPUT conditional?
> > >
> >
> > It has to be before the gpio_v2_line_config_flags_to_desc_flags() call,
> > as that modifies the desc flags, including the new polarity, so
> > polarity_change would then always be false :-).
> 
> I really don't see in the code how polarity_change value is used in
> FLAG_OUTPUT branch below.
> 

It isn't.  But desc->flags is modified before both - and so the
polarity_change initialization has to go before both SINCE IT TESTS
THE FLAGS.

Cheers,
Kent.