linux-kernel - Re: [PATCH V6] gpio: virtio: Add IRQ support

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Date:   Thu, 21 Oct 2021 13:02:32 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Arnd Bergmann <arnd@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        "Enrico Weigelt, metux IT consult" <info@...ux.net>,
        Viresh Kumar <vireshk@...nel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Bill Mills <bill.mills@...aro.org>,
        Alex Bennée <alex.bennee@...aro.org>,
        Cornelia Huck <cohuck@...hat.com>,
        "stratos-dev@...lists.linaro.org" <stratos-dev@...lists.linaro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Marc Zyngier <maz@...nel.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH V6] gpio: virtio: Add IRQ support

Hi Viresh,

On Thu, Oct 21, 2021 at 12:49 PM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 21-10-21, 12:07, Geert Uytterhoeven wrote:
> > On Thu, Oct 21, 2021 at 11:52 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > > The structure will get aligned to the size of largest element and each
> > > element will be aligned to itself. I don't see how this will break
> > > even in case of 32/64 bit communication.
> >
> > Structures are not aligned to the size of the largest element, but
> > to the largest alignment needed for each member.
>
> Right, I was talking in terms of the structures we have here for GPIO.
> The biggest member here (for any structure) is 32bits long, and
> compiler shouldn't add extra padding here.
>
> > This can be smaller than the size of the largest element.
> > E.g. alignof(long long) might be 4, not 8.
>
> Right.
>
> > And m68k aligns to two bytes at most.
>
> Interesting, I assumed that it will be 4bytes for 32 bit systems. So
> in case of m68k, we will see something like this ?
>
> struct foo {
>     u8 a;       // aligned to 2 bytes
>
>                 // padding of 1 byte
>
>     u32 b;      // aligned to 2 bytes
> }

Exactly.  And on CRIS (no longer supported by Linux), there won't
be any padding.

So I recommend to always add explicit padding, to make sure all
members are aligned naturally on all systems.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds