linux-kernel - Re: [PATCH v9 11/20] gpiolib: cdev: support GPIO_V2_LINE_SET_VALUES

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Date:   Thu, 24 Sep 2020 17:08:24 +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 11/20] gpiolib: cdev: support
 GPIO_V2_LINE_SET_VALUES_IOCTL

On Thu, Sep 24, 2020 at 11:21:05AM +0300, Andy Shevchenko wrote:
> On Thu, Sep 24, 2020 at 10:32 AM Kent Gibson <warthog618@...il.com> wrote:
> > On Wed, Sep 23, 2020 at 07:18:08PM +0300, Andy Shevchenko wrote:
> > > On Tue, Sep 22, 2020 at 5:36 AM Kent Gibson <warthog618@...il.com> wrote:
> > > >
> > > > Add support for the GPIO_V2_LINE_SET_VALUES_IOCTL.
> > >
> > > > +static long linereq_set_values_unlocked(struct linereq *lr,
> > > > +                                       struct gpio_v2_line_values *lv)
> > > > +{
> > > > +       DECLARE_BITMAP(vals, GPIO_V2_LINES_MAX);
> > > > +       struct gpio_desc **descs;
> > > > +       unsigned int i, didx, num_set;
> > > > +       int ret;
> > > > +
> > > > +       bitmap_zero(vals, GPIO_V2_LINES_MAX);
> > > > +       for (num_set = 0, i = 0; i < lr->num_lines; i++) {
> > > > +               if (lv->mask & BIT_ULL(i)) {
> > >
> > > Similar idea
> > >
> > > DECLARE_BITMAP(mask, 64) = BITMAP_FROM_U64(lv->mask);
> > >
> > > num_set = bitmap_weight();
> > >
> >
> > I had played with this option, but bitmap_weight() counts all
> > the bits set in the mask - which considers bits >= lr->num_lines.
> 
> Does it mean we have a bug in the code and no test case covered it?
> Because from the API I see the parameter nbits which should prevent
> this.
> 

Probably me being thick and using nbits as the size of the mask
rather than the number of bits of interest.

Cheers,
Kent.

> > So you would need to mask lv->mask before converting it to a bitmap.
> > (I'm ok with ignoring those bits in case userspace wants to be lazy and
> > use an all 1s mask.)
> 
> Can you confirm a bug in bitmap API? If it's so it's a serious one.
> 
> > But since we're looping over the bitmap anyway we may as well just
> > count as we go.
> >
> > > for_each_set_bit(i, mask, lr->num_lines)
> > >
> >
> > Yeah, that should work.  I vaguely recall trying this and finding it
> > generated larger object code, but I'll give it another try and if it
> > works out then include it in v10.
> 
> 
> -- 
> With Best Regards,
> Andy Shevchenko