netdev - Re: [PATCH net 1/2] net: bcmgenet: Fix sparse warnings in bcmgenet_put_tx

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Message-ID: <20180403164545.GK30522@ZenIV.linux.org.uk>
Date:   Tue, 3 Apr 2018 17:45:45 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     David Miller <davem@...emloft.net>
Cc:     f.fainelli@...il.com, netdev@...r.kernel.org, opendmb@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 1/2] net: bcmgenet: Fix sparse warnings in
 bcmgenet_put_tx_csum()

On Tue, Apr 03, 2018 at 12:33:05PM -0400, David Miller wrote:

> Yes Al, however the pattern choosen here is probably cheaper on little endian:
> 
> 	__beXX val = x;
> 	switch (val) {
> 	case htons(ETH_P_FOO):
> 	 ...
> 	}
> 
> This way only the compiler byte swaps the constants at compile time,
> no code is actually generated to do it.

That's not obvious, actually - depends upon how sparse the switch ends
up being.  You can easily lose more than a single byteswap insn on
worse cascase of comparisons.