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Message-ID: <07d36e94-aad4-a263-bf09-705ee1dd59ed@solarflare.com> Date: Mon, 29 Apr 2019 11:11:20 +0100 From: Edward Cree <ecree@...arflare.com> To: Nicholas Mc Guire <hofrat@...dl.org>, Jamal Hadi Salim <jhs@...atatu.com> CC: Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>, "David S. Miller" <davem@...emloft.net>, <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] net_sched: force endianness annotation On 28/04/2019 06:54, Nicholas Mc Guire wrote: > While the endiannes is being handled correctly sparse was unhappy with > the missing annotation as be16_to_cpu()/be32_to_cpu() expects a __be16 > respectively __be32. [...] > diff --git a/net/sched/em_cmp.c b/net/sched/em_cmp.c > index 1c8360a..3045ee1 100644 > --- a/net/sched/em_cmp.c > +++ b/net/sched/em_cmp.c > @@ -41,7 +41,7 @@ static int em_cmp_match(struct sk_buff *skb, struct tcf_ematch *em, > val = get_unaligned_be16(ptr); > > if (cmp_needs_transformation(cmp)) > - val = be16_to_cpu(val); > + val = be16_to_cpu((__force __be16)val); > break; There should probably be a comment here to explain what's going on. TBH it's probably a good general rule that any use of __force should have a comment explaining why it's needed. AFAICT, get_unaligned_be16(ptr) is (barring alignment) equivalent to be16_to_cpu(*(__be16 *)ptr). But then calling be16_to_cpu() again on val is bogus; it's already CPU endian. There's a distinct lack of documentation around as to the intended semantics of TCF_EM_CMP_TRANS, but it would seem either (__force u16)cpu_to_be16(val); (which preserves the existing semantics, that trans is a no-op on BE) or swab16(val); would make more sense. -Ed
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