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In-Reply-To: <87czfvrwsv.fsf@intel.com>
From:   Arnd Bergmann <arnd@arndb.de>
Date:   Mon, 30 May 2022 15:35:28 +0200
Message-ID: <CAK8P3a0HNOCOSx9UONw6gf1UUU8J4iAOmMFP8-DtJ3O7XAACsQ@mail.gmail.com>
Subject: Re: mainline build failure due to f1e4c916f97f ("drm/edid: add EDID
 block count and size helpers")
To:     Jani Nikula <jani.nikula@intel.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
        Russell King <linux@armlinux.org.uk>,
        Viresh Kumar <vireshk@kernel.org>,
        Shiraz Hashim <shiraz.linux.kernel@gmail.com>,
        =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= <ville.syrjala@linux.intel.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        SoC Team <soc@kernel.org>
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On Mon, May 30, 2022 at 3:10 PM Jani Nikula <jani.nikula@intel.com> wrote:
> >
> > I think in general, most __packed annotations we have in the kernel are
> > completely pointless because they do not change the structure layout on
> > any architecture but instead just make member access slower on
>
> Please explain.
>
> They are used quite a bit for parsing blob data, or
> serialization/deserialization, like in the EDID case at hand. Try
> removing __attribute__((packed)) from include/drm/drm_edid.h and see the
> sizeof(struct edid) on any architecture.

The annotations for edid are completely correct and necessary. However
other driver authors just slap __packed annotations on any structure
even if the layout is not fixed at all like:

struct my_driver_priv {
       struct device dev;
       u8 causes_misalignment;
       spinlock_t lock;
       atomic_t counter;
} __packed; /* this annotation is harmful because it breaks the atomics */

or if the annotation does not change the layout like

struct my_dma_descriptor {
     __le64 address;
     __le64 length;
} __packed; /* does not change layout but makes access slow on some
architectures */

       Arnd