From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ursula Braun Subject: RE: [PATCH net-next 2/3] qeth: Convert use of __constant_htons to htons Date: Thu, 20 Aug 2015 15:53:42 +0200 Message-ID: <1440078822.685.11.camel@BR9GV9YG.de.ibm.com> References: <1439972454-64347-1-git-send-email-ubraun@linux.vnet.ibm.com> <1439972454-64347-3-git-send-email-ubraun@linux.vnet.ibm.com> <063D6719AE5E284EB5DD2968C1650D6D1CB804CD@AcuExch.aculab.com> <1440071036.685.7.camel@BR9GV9YG.de.ibm.com> <063D6719AE5E284EB5DD2968C1650D6D1CB8054E@AcuExch.aculab.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <063D6719AE5E284EB5DD2968C1650D6D1CB8054E@AcuExch.aculab.com> Sender: netdev-owner@vger.kernel.org List-Archive: List-Post: To: David Laight Cc: "davem@davemloft.net" , "schwidefsky@de.ibm.com" , "heiko.carstens@de.ibm.com" , "netdev@vger.kernel.org" , "linux-s390@vger.kernel.org" , "ursula.braun@de.ibm.com" , Vaishali Thakkar List-ID: On Thu, 2015-08-20 at 11:51 +0000, David Laight wrote: > From: Ursula Braun [mailto:ubraun@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. Ursula