netdev - Re: Problems obtaining software TX timestamps

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Date:	Thu, 09 Sep 2010 10:26:06 +0200
From:	Ingo Kofler <ikofler@....uni-klu.ac.at>
To:	Richard Cochran <richardcochran@...il.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Subject: Re: Problems obtaining software TX timestamps

Richard Cochran wrote:
> On Wed, Sep 08, 2010 at 11:21:36PM +0200, Ingo Kofler wrote:
>   
>> Ok, for hardware timestamps it seems logically for me that the
>> driver has to support this. But what about the software timestamps -
>> do they also require support from the driver? Is there no generic
>> implementation in the networking stack?
>>     
>
> There was once a completely generic implementation, but the Tx path
> had issues. The generic Tx SW code was reverted in
>
> cd4d8fdad1f13205c769266dfa99015e226b6e07
>
> Now, there is a way to support this again, but it requires adding one
> line into each and every MAC driver. It is not very hard to do:
>
> linux/skbuff.h:1992
> /**
>  * skb_tx_timestamp() - Driver hook for transmit timestamping
>  *
>  * Ethernet MAC Drivers should call this function in their hard_xmit()
>  * function as soon as possible after giving the sk_buff to the MAC
>  * hardware, but before freeing the sk_buff.
>  *
>  * @skb: A socket buffer.
>  */
> static inline void skb_tx_timestamp(struct sk_buff *skb);
>
> I am already carrying patches for this privately. If you post a patch
> for your MAC, then you can be the first one to support this feature! ;^)
>   
Thank you very much for this clarification. Are there any drawbacks with 
this approach, e.g. performance issues? Honestly, I am just wondering 
why driver developers do not add this single line if it's that easy....

Ingo
> Richard
>
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>   

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