From mboxrd@z Thu Jan  1 00:00:00 1970
From: Bruce Richardson <bruce.richardson@intel.com>
Subject: Re: [PATCH v6 7/8] rte_mbuf.h: add and subtract
 explicitly to avoid promotion
Date: Mon, 21 May 2018 14:10:55 +0100
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To: Andy Green <andy@warmcat.com>
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On Mon, May 21, 2018 at 10:01:28AM +0800, Andy Green wrote:
>    /projects/lagopus/src/dpdk/build/include/rte_mbuf.h:
>     In function 'rte_pktmbuf_prepend':
>     /projects/lagopus/src/dpdk/build/include/rte_mbuf.h:
>     1908:17: warning: conversion from 'int' to 'uint16_t'
>     {aka 'short unsigned int'} may change value [-Wconversion]
>       m->data_off -= len;
>                      ^~~
>     m->data_off is a uint16_t
> 
>             uint16_t data_off;
> 
>     len (a uint16_t) is promoted to an int using -=.  Do the
>     subtraction explicitly and cast the result to uint16_t.
> 
>     -       m->data_off -= len;
>     +       m->data_off = (uint16_t)(m->data_off - len);
> 
>     The below += or -= changes are solving the same thing.
> 
>     /projects/lagopus/src/dpdk/build/include/rte_mbuf.h:
>     In function 'rte_pktmbuf_adj':
>     /projects/lagopus/src/dpdk/build/include/rte_mbuf.h:
>     1969:17: warning: conversion from 'int' to 'uint16_t'
>     {aka 'short unsigned int'} may change value [-Wconversion]
>       m->data_off += len;
>                      ^~~
>     /projects/lagopus/src/dpdk/build/include/rte_mbuf.h:
>     In function 'rte_pktmbuf_chain':
>     /projects/lagopus/src/dpdk/build/include/rte_mbuf.h:
>     2082:19: warning: conversion from 'int' to 'uint16_t'
>     {aka 'short unsigned int'} may change value [-Wconversion]
>       head->nb_segs += tail->nb_segs;
>                        ^~~~
>     Also uint16_t
> 
>             uint16_t nb_segs;         /**< Number of segments. */
> 
> Fixes: 08b563ffb1 ("mbuf: replace data pointer by an offset")
> Signed-off-by: Andy Green <andy@warmcat.com>
> ---
>  lib/librte_mbuf/rte_mbuf.h |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index a0423a548..beb104c69 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -1908,7 +1908,7 @@ static inline char *rte_pktmbuf_prepend(struct rte_mbuf *m,
>  	if (unlikely(len > rte_pktmbuf_headroom(m)))
>  		return NULL;
>  
> -	m->data_off -= len;
> +	m->data_off = (uint16_t)(m->data_off - len);
>  	m->data_len = (uint16_t)(m->data_len + len);
>  	m->pkt_len  = (m->pkt_len + len);
>  
Code change looks ok to me, again it wouldn't hurt to have a comment
explaining the absense of -=, but otherwise:

Acked-by: Bruce Richardson <bruce.richardson@intel.com>