linux-kernel - Re: [PATCH v2] include/linux/skbuff.h: using '(u16) ~0U' instead of '~0U'

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Date:	Mon, 3 Jun 2013 14:34:42 +0300
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	Chen Gang <gang.chen@...anux.com>
Cc:	edumazet@...gle.com, Pravin Shelar <pshelar@...ira.com>,
	Mel Gorman <mgorman@...e.de>,
	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	netdev <netdev@...r.kernel.org>,
	Ben Hutchings <bhutchings@...arflare.com>
Subject: Re: [PATCH v2] include/linux/skbuff.h: using '(u16) ~0U' instead of '~0U'

On Mon, Jun 3, 2013 at 2:24 PM, Chen Gang <gang.chen@...anux.com> wrote:
>
> Both 'transport_header' and 'mac_header' are u16, which are never equal
> to '~0U'.
>
> So need use '(u16) ~0U' instead of '~0U'.
>
> The related warning (with EXTRA_CFLAGS=-W ARCH=m68k for allmodconfig)
>   include/linux/skbuff.h:1587:2: warning: comparison is always true due to limited range of data type [-Wtype-limits]
>   ...
>
> Use meaningful macro instead of hard code number, and better to
> initialize 'skb->transport_header' in __alloc_skb_head(), too.

Looks okay. Couple of questions below.

> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -40,6 +40,8 @@
>  #define CHECKSUM_COMPLETE 2
>  #define CHECKSUM_PARTIAL 3
>
> +#define SKB_HEADER_UNSET_16    ((unsigned short) ~0U)

Isn't better to use the same type as used in the structure description?

> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -200,7 +200,8 @@ struct sk_buff *__alloc_skb_head(gfp_t gfp_mask, int node)
>         atomic_set(&skb->users, 1);
>
>  #ifdef NET_SKBUFF_DATA_USES_OFFSET
> -       skb->mac_header = (__u16) ~0U;
> +       skb->mac_header = SKB_HEADER_UNSET_16;
> +       skb->transport_header = SKB_HEADER_UNSET_16;

Is it correct to assign transport_header here as well?

--
With Best Regards,
Andy Shevchenko
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