On Thu, 21 Oct 2021 19:16:19 +0300
Eli Britstein [off-list ref] wrote:

On 10/21/2021 6:48 PM, Stephen Hemminger wrote:

quoted

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On Thu, 21 Oct 2021 11:51:30 +0300
Eli Britstein [off-list ref] wrote:
 

quoted

In rte_vlan_insert there is a casting of rte_pktmbuf_prepend returned
value to (struct rte_ether_hdr *), which causes cast-align warning when
using strict cast align flag with supporting gcc:
gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
CFLAGS="-Wcast-align=strict" make V=1 -C examples/l2fwd clean static

In file included from main.c:35:
/dpdk/build/include/rte_ether.h:370:7: warning: cast increases required
alignment of target type [-Wcast-align]
   370 |  nh = (struct rte_ether_hdr *)
       |       ^

As the code assumes correct alignment, add first a (void *) casting, to
avoid the warning.

Fixes: c974021a5949 ("ether: add soft vlan encap/decap")
Cc: stable@dpdk.org

Signed-off-by: Eli Britstein <redacted>
Acked-by: Olivier Matz <redacted>  

After cast to void * the second cast is not necessary.

         nh = (void *)rte_pktmbuf_prepend(...)

Ideally rte_pktmbuf_prepend() should return void * but that is
an API change.  

Removing the second cast, it is silently done anyway, as 'nh' is of type 
'struct rte_ether_hdr *'.

Going with this approach (I can also do it for patch 3/3), we can change 
rte_pktmbuf_prepend to return (void *), and let the applications using 
it do the silent cast.

What do you think?

Changing return type is an API change so it would need the whole
multistep process.

I overstated a little, it turns out the cast is necessary when header
is included by C++ code. C++ is pickier and doesn't allow void * to be
converted to other type by assignment.

Probably best to stick with what you originally proposed.
Gcc does have a bunch of alignment attribute types that could also fix this
but that gets even messier.