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

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Date:   Tue, 03 Apr 2018 12:33:05 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     viro@...IV.linux.org.uk
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()

From: Al Viro <viro@...IV.linux.org.uk>
Date: Tue, 3 Apr 2018 17:29:33 +0100

> On Mon, Apr 02, 2018 at 03:58:55PM -0700, Florian Fainelli wrote:
>> skb->protocol is a __be16 which we would be calling htons() against,
>> while this is not wrong per-se as it correctly results in swapping the
>> value on LE hosts, this still upsets sparse. Adopt a similar pattern to
>> what other drivers do and just assign ip_ver to skb->protocol, and then
>> use htons() against the different constants such that the compiler can
>> resolve the values at build time.
> 
> Fuck that and fuck other drivers sharing that "pattern".  It's not hard
> to remember:
> 	host-endian to net-endian short is htons
> 	host-endian to net-endian long is htonl
> 	net-endian to host-endian short is ntohs
> 	net-endian to host-endian long is ntohl
> 
> That's *it*.  No convoluted changes needed, just use the right primitive.
> You are turning trivial one-liners ("conversions between host- and net-endian
> are the same in both directions, so the current code works even though we
> are using the wrong primitive, confusing the readers.  Use the right one")
> which are absolutely self-contained and safe into much more massive changes
> that are harder to follow and which are *not* self-contained at all.
> 
> Don't do it.

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.