RE: [PATCH] carl9170: fix struct alignment conflict

From: Arnd Bergmann

Sent: 04 February 2021 16:29

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.

I think I'd add compile-time assert that some of these structures
are exactly the expected size.
Then look at removing the outer packed/aligned attributes
and just putting the attribute on the 16/32 bit member(s)
that themselves might be misaligned.
Much the way that the 32bit aligned 64bit values are handled
in the x86 compat code in x86-64.

	David

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