Virtualization - Oct 2021 - [PATCH V6] gpio: virtio: Add IRQ support

Hi Viresh,

On Thu, Oct 21, 2021 at 11:52 AM Viresh Kumar <viresh.kumar at linaro.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.
This can be smaller than the size of the largest element.
E.g. alignof(long long) might be 4, not 8.  And m68k aligns to
two bytes at most.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at
linux-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

On 21-10-21, 12:07, Geert Uytterhoeven wrote:
> On Thu, Oct 21, 2021 at 11:52 AM Viresh Kumar <viresh.kumar at
linaro.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