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SubjectRe: [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@gmail.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@gmail.com> wrote:
> > > > On Tue, Sep 22, 2020 at 5:35 AM Kent Gibson <warthog618@gmail.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.

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