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

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Message-ID: <20211021104950.nhvify5lo7y5fdfh@vireshk-i7>
Date:   Thu, 21 Oct 2021 16:19:50 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.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

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
}

-- 
viresh