netdev - Re: [PATCH] carl9170: fix struct alignment conflict

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Message-ID: <6ec7c9f0-51c6-b16f-9252-2dad7308f6fc@gmail.com>
Date:   Thu, 4 Feb 2021 18:22:01 +0100
From:   Christian Lamparter <chunkeey@...il.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        Christian Lamparter <chunkeey@...glemail.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] carl9170: fix struct alignment conflict

On 04/02/2021 17:29, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> Multiple structures in the carl9170 driver have alignment
> impossible alignment constraints that gcc warns about when
> building with 'make W=1':
> 
> drivers/net/wireless/ath/carl9170/fwcmd.h:243:2: warning: alignment 1 of 'union <anonymous>' is less than 4 [-Wpacked-not-aligned]
> drivers/net/wireless/ath/carl9170/wlan.h:373:1: warning: alignment 1 of 'struct ar9170_rx_frame_single' is less than 2 [-Wpacked-not-aligned]
> 
> In the carl9170_cmd structure, multiple members that have an explicit
> alignment requirement of four bytes are added into a union with explicit
> byte alignment, but this in turn is part of a structure that also has
> four-byte alignment.
> 
> In the wlan.h header, multiple structures contain a ieee80211_hdr member
> that is required to be two-byte aligned to avoid alignmnet faults when
> processing network headers, but all members are forced to be byte-aligned
> using the __packed tag at the end of the struct definition.
> 
> In both cases, leaving out the packing does not change the internal
> layout of the structure but changes the alignment constraint of the
> structure itself.
> 
> Change all affected structures to only apply packing where it does
> not violate the alignment requirement of the contained structure.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Acked-by: Christian Lamparter <chunkeey@...il.com>

I've also applied this patch and a previous one dealing with VLAs to the firmware
source at <https://github.com/chunkeey/carl9170fw>. I did a quick rebuilt and
the same binary firmware was produced, so I think what's left is to update those
shared firmware/driver headers some day...

Cheers,
Christian