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Subject: Re: mainline build failure due to f1e4c916f97f ("drm/edid: add EDID
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To:     Arnd Bergmann <arnd@arndb.de>,
        Linus Torvalds <torvalds@linux-foundation.org>
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        Ayush Sawal <ayush.sawal@chelsio.com>,
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        Jani Nikula <jani.nikula@intel.com>,
        Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
        Russell King <linux@armlinux.org.uk>,
        Viresh Kumar <vireshk@kernel.org>,
        Shiraz Hashim <shiraz.linux.kernel@gmail.com>,
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        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
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        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        SoC Team <soc@kernel.org>
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On 2022/06/02 16:38, Arnd Bergmann wrote:
>> But let's cc the tomoyo and chelsio people.
> 
> I think both of them work because the structures are always
> embedded inside of larger structures that have at least word
> alignment. This is the thing I was looking for, and the
> __packed attribute was added in error, most likely copied
> from somewhere else.

The __packed in "struct tomoyo_shared_acl_head" is to embed next
naturally-aligned member of a larger struct into the bytes that
would have been wasted if __packed is not specified. For example,

struct tomoyo_shared_acl_head {
        struct list_head list;
        atomic_t users;
} __packed;

struct tomoyo_condition {
        struct tomoyo_shared_acl_head head;
        u32 size; /* Memory size allocated for this entry. */
	(...snipped...)
};

saves 4 bytes on 64 bits build.

If the next naturally-aligned member of a larger struct is larger than
the bytes that was saved by __packed, the saved bytes will be unused.