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 Syrjälä" <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")
Date: Mon, 30 May 2022 17:08:11 +0300 [thread overview]
Message-ID: <874k17ru44.fsf@intel.com> (raw)
In-Reply-To: <CAK8P3a0HNOCOSx9UONw6gf1UUU8J4iAOmMFP8-DtJ3O7XAACsQ@mail.gmail.com>
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
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2022-05-30 14:40 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <YpCUzStDnSgQLNFN@debian>
[not found] ` <CAHk-=wg0uGAX5DYZq+tY2KeUAR8DtR91YE1y9CkPMKkKOyE4jg@mail.gmail.com>
[not found] ` <CADVatmNGPbSdRNQuwJEWAaPtqb3vBYRjvsuBpoRUnhEHj=X5GQ@mail.gmail.com>
[not found] ` <CAHk-=wisQd8yiPX=SsK3eFiakKo713hq4SyqPWsJ-oyAmLFefQ@mail.gmail.com>
[not found] ` <YpIR67FMtTGCwARZ@debian>
[not found] ` <CAHk-=wjuyHE=1wLgHncub8FfgeyYqfWYsy4-YrhAvq9991h_Aw@mail.gmail.com>
2022-05-28 18:08 ` mainline build failure due to f1e4c916f97f ("drm/edid: add EDID block count and size helpers") Linus Torvalds
2022-05-28 18:58 ` Arnd Bergmann
2022-05-28 20:31 ` Linus Torvalds
2022-05-28 21:08 ` Arnd Bergmann
2022-05-30 9:31 ` Jani Nikula
2022-05-30 9:33 ` Jani Nikula
2022-05-30 12:43 ` Arnd Bergmann
2022-05-30 13:10 ` Jani Nikula
2022-05-30 13:35 ` Arnd Bergmann
2022-05-30 14:08 ` Jani Nikula [this message]
2022-05-30 14:26 ` Arnd Bergmann
2022-05-31 6:26 ` Julia Lawall
2022-05-31 8:04 ` Arnd Bergmann
2022-05-31 16:41 ` Linus Torvalds
2022-06-01 22:28 ` Keisuke Nishimura
2022-06-02 1:08 ` Linus Torvalds
2022-06-02 7:38 ` Arnd Bergmann
2022-06-02 11:21 ` Tetsuo Handa
2022-06-02 12:11 ` Arnd Bergmann
2022-06-02 13:18 ` Ard Biesheuvel
2022-06-02 12:19 ` Christoph Hellwig
2022-06-06 10:51 ` Keisuke Nishimura
2022-05-30 16:56 ` Russell King (Oracle)
2022-05-30 16:54 ` Russell King (Oracle)
2022-05-30 16:53 ` Russell King (Oracle)
2022-05-28 20:32 ` Russell King (Oracle)
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