netdev - Re: [RFCv2 bluetooth-next 2/3] ipv6: add ipv6_addr_prefix

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Date:	Thu, 26 Nov 2015 12:11:07 +0100
From:	Stefan Schmidt <stefan@....samsung.com>
To:	Alexander Aring <alex.aring@...il.com>
Cc:	linux-wpan@...r.kernel.org, linux-bluetooth@...r.kernel.org,
	netdev@...r.kernel.org, kernel@...gutronix.de, mcr@...delman.ca,
	lukasz.duda@...dicsemi.no, martin.gergeleit@...rm.de,
	"David S . Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [RFCv2 bluetooth-next 2/3] ipv6: add ipv6_addr_prefix_cpy

Hello.

On 25/11/15 19:17, Alexander Aring wrote:
> On Wed, Nov 25, 2015 at 05:42:52PM +0100, Stefan Schmidt wrote:
>> Hello.
>>
>> On 17/11/15 23:33, Alexander Aring wrote:
>>> This patch adds a static inline function ipv6_addr_prefix_cpy which
>>> copies a ipv6 address prefix(argument pfx) into the ipv6 address prefix.
>>> The prefix len is given by plen as bits. This function mainly based on
>>> ipv6_addr_prefix which copies one address prefix from address into a new
>>> ipv6 address destination and zero all other address bits.
>>>
>>> The difference is that ipv6_addr_prefix_cpy don't get a prefix from an
>>> ipv6 address, it sets a prefix to an ipv6 address with keeping other
>>> address bits. The use case is for context based address compression
>>> inside 6LoWPAN IPHC header which keeping ipv6 prefixes inside a context
>>> table to lookup address-bits without sending them.
>>>
>>> Cc: David S. Miller<davem@...emloft.net>
>>> Cc: Alexey Kuznetsov<kuznet@....inr.ac.ru>
>>> Cc: James Morris<jmorris@...ei.org>
>>> Cc: Hideaki YOSHIFUJI<yoshfuji@...ux-ipv6.org>
>>> Cc: Patrick McHardy<kaber@...sh.net>
>>> Signed-off-by: Alexander Aring<alex.aring@...il.com>
>>> ---
>>>   include/net/ipv6.h | 15 +++++++++++++++
>>>   1 file changed, 15 insertions(+)
>>>
>>> diff --git a/include/net/ipv6.h b/include/net/ipv6.h
>>> index e1a10b0..9d38fc2 100644
>>> --- a/include/net/ipv6.h
>>> +++ b/include/net/ipv6.h
>>> @@ -382,6 +382,21 @@ static inline void ipv6_addr_prefix(struct in6_addr *pfx,
>>>   		pfx->s6_addr[o] = addr->s6_addr[o] & (0xff00 >> b);
>>>   }
>>> +static inline void ipv6_addr_prefix_cpy(struct in6_addr *addr,
>>> +					const struct in6_addr *pfx,
>>> +					int plen)
>>> +{
>> Naming it ipv6_addr_prefix_cop, as Sergei suggested, would be easier to
>> read.
> ok.
>
>>> +	/* caller must guarantee 0 <= plen <= 128 */
>>> +	int o = plen >> 3,
>>> +	    b = plen & 0x7;
>>> +
>> Any reason you are not doing the memset here before memcpy like it is done
>> in ipv6_addr_prefi()?
> ipv6_addr_prefix is more a "getter" for a prefix from an address. The
> destination address (which should be the result) will memset the whole
> address before copy the prefix from address to it.

Ah, right. Missed that part.

> THe ipv6_addr_prefix_copy will set a prefix given by second parameter
> pfx and set the prefix to the address (given by first parameter).
>
> Maybe the ipv6_addr_prefix could call:
>
> 1. memset(pfx, sizeof...)
> 2. ipv6_addr_prefix_copy(pfx, addr, plen);
>
> but ipv6_addr_prefix_copy need to care about other bits which are set at
> destination address, if (plen/8 != 0). I don't want to touch
> ipv6_addr_prefix function and slow-down some mechanism.

No need to change this from my side. I just missed the memset being 
handled in the destination address.

With the name change to _copy you can add my

Reviewed-by: Stefan Schmidt <stefan@....samsung.com>

regards
Stefan Schmidt
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