netdev - RE: [PATCH net-next 2/3] qeth: Convert use of __constant

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Date:	Thu, 20 Aug 2015 10:46:43 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Ursula Braun' <ubraun@...ux.vnet.ibm.com>,
	"davem@...emloft.net" <davem@...emloft.net>
CC:	"schwidefsky@...ibm.com" <schwidefsky@...ibm.com>,
	"heiko.carstens@...ibm.com" <heiko.carstens@...ibm.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
	"ursula.braun@...ibm.com" <ursula.braun@...ibm.com>,
	Vaishali Thakkar <vthakkar1994@...il.com>
Subject: RE: [PATCH net-next 2/3] qeth: Convert use of __constant_htons to
 htons

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

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