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From:   Julia Lawall <Julia.Lawall@inria.fr>
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Subject: Re: mainline build failure due to f1e4c916f97f ("drm/edid: add EDID block count and size helpers")
Date:   Tue, 31 May 2022 07:26:53 +0100
Message-Id: <91E67F46-A3C7-4159-9E0C-C6C6306F3669@inria.fr>
References: <CAK8P3a2Zg2QDS1_Ysn8-Zqqd+K7bbTFS7JV7gPabp6nvPiKaog@mail.gmail.com>
Cc:     Jani Nikula <jani.nikula@intel.com>,
        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?Q?Ville_Syrj=C3=A4l=C3=A4?= <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>
In-Reply-To: <CAK8P3a2Zg2QDS1_Ysn8-Zqqd+K7bbTFS7JV7gPabp6nvPiKaog@mail.gmail.com>
To:     Arnd Bergmann <arnd@arndb.de>
Precedence: bulk


> On 30 May 2022, at 15:27, Arnd Bergmann <arnd@arndb.de> wrote:
>=20
> =EF=BB=BFOn Mon, May 30, 2022 at 4:08 PM Jani Nikula <jani.nikula@intel.co=
m> wrote:
>>> On Mon, 30 May 2022, Arnd Bergmann <arnd@arndb.de> wrote:
>>> 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 *=
/
>>=20
>> 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. ;)
>>=20
>> Cc: Julia.
>=20
> I think one would first have to make a list of data types that are not
> meant to be in a packed structure. It could be a good start to
> search for any packed aggregates with a pointer, atomic_t or spinlock_t
> in them, but there are of course many more types that you won't
> find in hardware structures.
>=20
>>> or if the annotation does not change the layout like
>>>=20
>>> struct my_dma_descriptor {
>>>     __le64 address;
>>>     __le64 length;
>>> } __packed; /* does not change layout but makes access slow on some
>>> architectures */
>>=20
>> Why is this the case, though? I'd imagine the compiler could figure this
>> out.
>=20
> When you annotate the entire structure as __packed without an
> extra __aligned() annotation, the compiler has to assume that the
> structure itself is unaligned as well. On many of the older architectures,=

> this will result in accessing the values one byte at a time. Marking
> the structure as "__packed __aligned(8)" instead would be harmless.
>=20
> When I have a structure with a few misaligned members, I generally
> prefer to only annotate the members that are not naturally aligned,
> but this approach is not very common.

Searching for specific types in a packed structure would be easy.

Coccinelle could duplicate the structure without the packed and see if any o=
ffsets change, using build bug on, but that would be architecture specific s=
o maybe not useful.

Julia


>         Arnd