Regarding net_device_ops

[Rami Rosen]

Hi,
Sorry, I of course discuss here your previous question about SKB
Regards,
Rami

On Sat, May 3, 2014 at 1:34 PM, Rami Rosen <roszenrami at gmail.com> wrote:
> Hi, Pranay,
>
> First, let's assume that you are talking about IPv4, though you did not
> mention it explicitly. (The principles in IPv6 are quite similar, though)
>
> A packet is sent out in the usual case with the ip_queue_xmit() method.
> The  ip_queue_xmit() method calls the ip_route_output_ports() method
> in order to perform a lookup in the IPv4 routing tables.
>
> see: http://lxr.free-electrons.com/source/net/ipv4/ip_output.c#L352
>
> The results of this IPv4 routing lookup determines on which network device
> (net_device) the packet will be sent. You should look at the code of
> ip_route_output_ports() method in net/ipv4/route.c in order to
> understand the IPv4 routing subsystem and the IPv4 routing lookup.
>
> Packets can, under certain circumstances, be sent by the ip_send_skb()
> method, but this happens when the flow (which consists also of the
> net_device to be used) is known before.
>
> Best Regards,
> Rami Rosen
> http://ramirose.wix.com/ramirosen
>
>
> On Sat, May 3, 2014 at 12:43 PM, Pranay Srivastava <pranjas at gmail.com> wrote:
>> Hi
>>
>> Referring to Documentation/networking/netdevices.txt
>>
>> a)
>>
>> ndo->ndo_start_xmit if written "as is" will be thread safe? [Correct?]
>> NETIF_F_LLTX is not set only then this is true.
>>
>> But dev_queue_xmit(skb) doesn't seem to take any lock, neither does
>> dev_hard_start_xmit. Please let me know if i'm wrong and where is this
>> lock taken before calling dev_queue_xmit/dev_hard_start_xmit.
>>
>>
>> b)
>>
>> Why and when would I want to use rtnl_lock/unlock? Is it like
>> registering multiple device simultaneously?
>>
>>
>> c)
>>
>> ndo->ndo_start_xmit is supposed to start the transmission of the skb.
>> However I've a doubt
>>
>> ---> Suppose that there's no room left for the incoming skb in the
>> device buffer, so in that case it would be ideal to call
>> netif_stop_queue. At this the the skb already with the ndo_start_xmit
>> will not be retried again[Correct?]
>>
>> ----> Is ndo_start_xmit required to wait in this case? However if the
>> above spin_lock in a) was taken then this would be a waste. So is it
>> possible that this skb is "deffered" to whenever there's room and
>> return NETDEV_TX_OK? But if we do this we are telling the application
>> that it's packet was delivered so in case there was a timeout set by
>> the application on a reply it would again send us the same packet. So
>> instead of "deferring" is it good to "drop" ?
>>
>>
>>
>> --
>>         ---P.K.S
>>
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