On Thu, 2015-08-20 at 11:51 +0000, David Laight wrote: > From: Ursula Braun [mailto:ubr...@linux.vnet.ibm.com] > > Sent: 20 August 2015 12:44 > > On Thu, 2015-08-20 at 10:46 +0000, David Laight wrote: > > > From: Ursula Braun > > > > Sent: 19 August 2015 09:21 > > > > In little endian cases, the macro htons unfolds to __swab16 which > > > > provides special case for constants. In big endian cases, > > > > __constant_htons and htons expand directly to the same expression. > > > > So, replace __constant_htons with htons with the goal of getting > > > > rid of the definition of __constant_htons completely. > > > ... > > > > diff --git a/drivers/s390/net/qeth_l3_main.c > > > > b/drivers/s390/net/qeth_l3_main.c > > > > index 70eb2f6..ecfe622 100644 > > > > --- a/drivers/s390/net/qeth_l3_main.c > > > > +++ b/drivers/s390/net/qeth_l3_main.c > > > > @@ -1887,13 +1887,13 @@ static inline int qeth_l3_rebuild_skb(struct > > > > qeth_card *card, > > > > case QETH_CAST_MULTICAST: > > > > switch (prot) { > > > > #ifdef CONFIG_QETH_IPV6 > > > > - case __constant_htons(ETH_P_IPV6): > > > > + case htons(ETH_P_IPV6): > > > > > > I didn't think htons() was 'constant enough' to be used as a case label. > > > > > > Using byteswapped constants in a case statement can change it from being > > > implemented as a jump table to a branch tree. > > > This might be more expensive than byteswapping the value (even on systems > > > that don't have cheap byteswap instructions). > > > > > > David > > > > > For big endian systems both __constant_htons(x) and htons(x) are > > resolved to ((__force __be16)(__u16)(x)). Thus I do not see a reason to > > reject the patch proposal from Vaishali Thakkar. > > Look at a little-endian one (eg amd64). > I think you'll find a C ?: expression that uses __builtin_constant() to > select between an expression the compiler can evaluate and a call to an > inline function that uses some appropriate asm. > The latter isn't a compile-time constant so can't be used as a case > label or as an initialiser. > > David > > qeth is an s390-driver, and s390 is a big-endian architecture. Thus arguments valid for little-endian do not apply to qeth-code.
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