linux-kernel - Re: mainline build failure due to f1e4c916f97f ("drm/edid: add EDID block count and size helpers")

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Date:   Thu, 2 Jun 2022 14:11:13 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Keisuke Nishimura <keisuke.nishimura@...ia.fr>,
        Kentaro Takeda <takedakn@...data.co.jp>,
        Ayush Sawal <ayush.sawal@...lsio.com>,
        Vinay Kumar Yadav <vinay.yadav@...lsio.com>,
        Rohit Maheshwari <rohitm@...lsio.com>,
        Julia Lawall <Julia.Lawall@...ia.fr>,
        Jani Nikula <jani.nikula@...el.com>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Viresh Kumar <vireshk@...nel.org>,
        Shiraz Hashim <shiraz.linux.kernel@...il.com>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        SoC Team <soc@...nel.org>
Subject: Re: mainline build failure due to f1e4c916f97f ("drm/edid: add EDID
 block count and size helpers")

On Thu, Jun 2, 2022 at 1:21 PM Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
> On 2022/06/02 16:38, Arnd Bergmann wrote:
> >> But let's cc the tomoyo and chelsio people.
> >
> > I think both of them work because the structures are always
> > embedded inside of larger structures that have at least word
> > alignment. This is the thing I was looking for, and the
> > __packed attribute was added in error, most likely copied
> > from somewhere else.
>
> The __packed in "struct tomoyo_shared_acl_head" is to embed next
> naturally-aligned member of a larger struct into the bytes that
> would have been wasted if __packed is not specified. For example,
>
> struct tomoyo_shared_acl_head {
>         struct list_head list;
>         atomic_t users;
> } __packed;
>
> struct tomoyo_condition {
>         struct tomoyo_shared_acl_head head;
>         u32 size; /* Memory size allocated for this entry. */
>         (...snipped...)
> };
>
> saves 4 bytes on 64 bits build.
>
> If the next naturally-aligned member of a larger struct is larger than
> the bytes that was saved by __packed, the saved bytes will be unused.

Ok, got it. I think as gcc should still be able to always figure out the
alignment when accessing the atomic, without ever falling back
to byte access on an atomic_get() or atomic_set().

To be on the safe side, I would still either move the __packed attribute
to the 'list' member, or make the structure '__aligned(4)'.

       Arnd