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From:   Bartosz Golaszewski <bgolaszewski@baylibre.com>
Date:   Wed, 9 Sep 2020 11:54:51 +0200
Message-ID: <CAMpxmJVff+=qeLCottELYTF2yzykyvaoP=jxZ-vaNgCL-MA2CQ@mail.gmail.com>
Subject: Re: [PATCH v7 07/20] gpiolib: cdev: support GPIO_V2_GET_LINE_IOCTL
 and GPIO_V2_LINE_GET_VALUES_IOCTL
To:     Kent Gibson <warthog618@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        linux-gpio <linux-gpio@vger.kernel.org>,
        Linus Walleij <linus.walleij@linaro.org>
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On Wed, Sep 9, 2020 at 11:49 AM Kent Gibson <warthog618@gmail.com> wrote:
>
> On Wed, Sep 09, 2020 at 11:41:35AM +0200, Bartosz Golaszewski wrote:
> > On Wed, Sep 9, 2020 at 11:35 AM Kent Gibson <warthog618@gmail.com> wrote:
> > >
> > > On Wed, Sep 09, 2020 at 11:26:00AM +0200, Bartosz Golaszewski wrote:
> > > > On Wed, Sep 9, 2020 at 11:24 AM Kent Gibson <warthog618@gmail.com> wrote:
> > > > >
> > > > > On Sat, Sep 05, 2020 at 09:35:36PM +0800, Kent Gibson wrote:
> > > > > > Add support for requesting lines using the GPIO_V2_GET_LINE_IOCTL, and
> > > > > > returning their current values using GPIO_V2_LINE_GET_VALUES_IOCTL.
> > > > > >
> > > > > > The struct linereq implementation is based on the v1 struct linehandle
> > > > > > implementation.
> > > > > >
> > > > > > Signed-off-by: Kent Gibson <warthog618@gmail.com>
> > > > > > ---
> > > > > >
> > > > >
> > > > > [snip]
> > > > >
> > > > > >               if (copy_from_user(&offset, ip, sizeof(offset)))
> > > > > >                       return -EFAULT;
> > > > > > @@ -1104,6 +1505,25 @@ int gpiolib_cdev_register(struct gpio_device *gdev, dev_t devt)
> > > > > >                MAJOR(devt), gdev->id);
> > > > > >
> > > > > >       return 0;
> > > > > > +     /*
> > > > > > +      * array sizes must ensure 64-bit alignment and not create holes in
> > > > > > +      * the struct packing.
> > > > > > +      */
> > > > > > +     BUILD_BUG_ON(IS_ALIGNED(GPIO_V2_LINES_MAX, 2));
> > > > > > +     BUILD_BUG_ON(IS_ALIGNED(GPIO_MAX_NAME_SIZE, 8));
> > > > > > +
> > > > > > +     /*
> > > > > > +      * check that uAPI structs are 64-bit aligned for 32/64-bit
> > > > > > +      * compatibility
> > > > > > +      */
> > > > > > +     BUILD_BUG_ON(IS_ALIGNED(sizeof(struct gpio_v2_line_attribute), 8));
> > > > > > +     BUILD_BUG_ON(IS_ALIGNED(sizeof(struct gpio_v2_line_config_attribute), 8));
> > > > > > +     BUILD_BUG_ON(IS_ALIGNED(sizeof(struct gpio_v2_line_config), 8));
> > > > > > +     BUILD_BUG_ON(IS_ALIGNED(sizeof(struct gpio_v2_line_request), 8));
> > > > > > +     BUILD_BUG_ON(IS_ALIGNED(sizeof(struct gpio_v2_line_info), 8));
> > > > > > +     BUILD_BUG_ON(IS_ALIGNED(sizeof(struct gpio_v2_line_info_changed), 8));
> > > > > > +     BUILD_BUG_ON(IS_ALIGNED(sizeof(struct gpio_v2_line_event), 8));
> > > > > > +     BUILD_BUG_ON(IS_ALIGNED(sizeof(struct gpio_v2_line_values), 8));
> > > > > >  }
> > > > > >
> > > > >
> > > > > A couple of things here - these should all be !IS_ALIGNED.
> > > > > And the BUILD_BUG_ON gets compiled out, and so doesn't fail, if they are
> > > > > after the return.
> > > > >
> > > > > How would you like that fixed - v8 or a patch once v7 is in?
> > > > >
> > > > > Cheers,
> > > > > Kent.
> > > >
> > > > v8 please. Why is it compiled out though? Does it need some config option?
> > > >
> > >
> > > Not sure - haven't looked into it.  I only noticed the condition was
> > > inverted in passing, and when I flipped it it still compiled.
> > > Moving the return to the end of the function made them all fail,
> > > as they should if IS_ALIGNED is true.
> > >
> > > Having BUILD_BUG_ON being able to compile out quietly is a problem in
> > > itself.  Nothing special in my setup that I am aware of.
> > >
> > > Cheers,
> > > Kent.
> >
> > From include/linux/compiler_types.h:
> >
> > 295 #ifdef __OPTIMIZE__
> > 296 # define __compiletime_assert(condition, msg, prefix, suffix)           \
> > 297         do {                                                            \
> > 298                 extern void prefix ## suffix(void)
> > __compiletime_error(msg); \
> > 299                 if (!(condition))                                       \
> > 300                         prefix ## suffix();                             \
> > 301         } while (0)
> > 302 #else
> > 303 # define __compiletime_assert(condition, msg, prefix, suffix) do {
> > } while (0)
> > 304 #endif
> >
> > __OPTIMIZE__ is a predefined macro. I'm not sure about your setup but
> > it it's defined for me in all my yocto SDK builds and BUILD_BUG_ON(1)
> > fails as expected.
> >
>
> Even when placed after the return, as was the case here?
>
> Kent.

No of course not, I put it into a void function. Sorry for the noise.

I'm not sure how to disable optimizations for a macro.

For now putting it before return should be fine.

Bart