linux-kernel - Re: [PATCH v2 10/11] gpiolib: add new ioctl() for monitoring changes in line info

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Message-ID: <CAMRc=Mcy=Q+9Eb6mb5JEq+CCbxgBY1CfTDsYj1Rt9bcLXgeY=g@mail.gmail.com>
Date:   Thu, 19 Dec 2019 14:05:19 +0100
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Kent Gibson <warthog618@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 10/11] gpiolib: add new ioctl() for monitoring changes
 in line info

wt., 10 gru 2019 o 18:00 Andy Shevchenko <andy.shevchenko@...il.com> napisał(a):
>
> > On a different note: why would endianness be an issue here? 32-bit
> > variables with 64-bit alignment should still be in the same place in
> > memory, right?
>
> With explicit padding, yes.
>
> > Any reason not to use __packed for this structure and not deal with
> > this whole compat mess?
>
> Have been suggested that explicit padding is better approach.
> (See my answer to Kent)
>
> > I also noticed that my change will only allow user-space to read one
> > event at a time which seems to be a regression with regard to the
> > current implementation. I probably need to address this too.
>
> Yes, but we have to have ABI v2 in place.

Hi Andy,

I was playing with some ideas for the new ABI and noticed that on
64-bit architecture the size of struct gpiochip_info is reported to be
68 bytes, not 72 as I would expect. Is implicit alignment padding not
applied to a struct if there's a non-64bit-aligned 32-bit field at the
end of it? Is there something I'm missing here?

Bart