Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
From:   Jani Nikula <jani.nikula@intel.com>
To:     Arnd Bergmann <arnd@arndb.de>
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>,
        Ville =?utf-8?B?U3lyasOkbMOk?= <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>, Julia Lawall <Julia.Lawall@inria.fr>
Subject: Re: mainline build failure due to f1e4c916f97f ("drm/edid: add EDID
 block count and size helpers")
In-Reply-To: <CAK8P3a0HNOCOSx9UONw6gf1UUU8J4iAOmMFP8-DtJ3O7XAACsQ@mail.gmail.com>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
References: <YpCUzStDnSgQLNFN@debian>
 <CAHk-=wg0uGAX5DYZq+tY2KeUAR8DtR91YE1y9CkPMKkKOyE4jg@mail.gmail.com>
 <CADVatmNGPbSdRNQuwJEWAaPtqb3vBYRjvsuBpoRUnhEHj=X5GQ@mail.gmail.com>
 <CAHk-=wisQd8yiPX=SsK3eFiakKo713hq4SyqPWsJ-oyAmLFefQ@mail.gmail.com>
 <YpIR67FMtTGCwARZ@debian>
 <CAHk-=wjuyHE=1wLgHncub8FfgeyYqfWYsy4-YrhAvq9991h_Aw@mail.gmail.com>
 <CAHk-=wi_hJV0V=Ecg2dzbe2P_H1XKTu6VP_AtCH6u=tis31ayg@mail.gmail.com>
 <CAK8P3a0-QyOQiieEvM0yQb43XbCtPmeao8UvoAsdFnjCxYPz7Q@mail.gmail.com>
 <CAHk-=whfmwzjF4eBPYS6pHFqHVzJF3m=2h=gRWSRyHks8V=ABA@mail.gmail.com>
 <87a6aztli2.fsf@intel.com> <877d63tleq.fsf@intel.com>
 <CAK8P3a0-S77QLR1dK3NT6ot7JTAD5AdojAZJr-Xi112-v5EOdw@mail.gmail.com>
 <87czfvrwsv.fsf@intel.com>
 <CAK8P3a0HNOCOSx9UONw6gf1UUU8J4iAOmMFP8-DtJ3O7XAACsQ@mail.gmail.com>
Date:   Mon, 30 May 2022 17:08:11 +0300
Message-ID: <874k17ru44.fsf@intel.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Mon, 30 May 2022, Arnd Bergmann <arnd@arndb.de> wrote:
> 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:

Right. Thanks for the examples.

> 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 */

I wonder if this is something that could be caught with coccinelle. Or
sparse. Are there any cases where this combo is necessary? (I can't
think of any, but it's a low bar. ;)

Cc: Julia.

> 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 */

Why is this the case, though? I'd imagine the compiler could figure this
out.


BR,
Jani.

-- 
Jani Nikula, Intel Open Source Graphics Center