From: Jani Nikula <jani.nikula@intel.com>
To: Arnd Bergmann <arnd@arndb.de>
Subject: Re: mainline build failure due to f1e4c916f97f ("drm/edid: add EDID
 block count and size helpers")
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Date: Mon, 30 May 2022 17:08:11 +0300
Message-ID: <874k17ru44.fsf@intel.com>
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Cc: Linux ARM <linux-arm-kernel@lists.infradead.org>,
 Arnd Bergmann <arnd@arndb.de>, David Airlie <airlied@linux.ie>,
 Viresh Kumar <vireshk@kernel.org>, Russell King <linux@armlinux.org.uk>,
 Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
 Julia Lawall <Julia.Lawall@inria.fr>,
 Linus Torvalds <torvalds@linux-foundation.org>, SoC Team <soc@kernel.org>,
 dri-devel <dri-devel@lists.freedesktop.org>,
 Thomas Zimmermann <tzimmermann@suse.de>,
 Shiraz Hashim <shiraz.linux.kernel@gmail.com>,
 Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Errors-To: dri-devel-bounces@lists.freedesktop.org
Sender: "dri-devel" <dri-devel-bounces@lists.freedesktop.org>

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