netdev - RE: [RFC PATCH 00/13] hardware time stamping + igb example implementation

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Date:	Thu, 13 Nov 2008 15:53:27 +0000
From:	"Ohly, Patrick" <patrick.ohly@...el.com>
To:	Oliver Hartkopp <oliver@...tkopp.net>,
	Andi Kleen <ak@...ux.intel.com>
CC:	Eric Dumazet <dada1@...mosbay.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Octavian Purdila <opurdila@...acom.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Ingo Oeser <netdev@...eo.de>,
	"Ronciak, John" <john.ronciak@...el.com>
Subject: RE: [RFC PATCH 00/13] hardware time stamping + igb example
 implementation

Oliver Hartkopp wrote:
>Andi Kleen wrote:
>>Eric Dumazet wrote:
>>> I threw a "crazy idea", that can be changed if necessary, say with a 
>>> cookie
>>> that identifies the slot in NIC driver structure. O(1) lookup if 
>>> really needed.
>>
>> I think "crazy" describes it well because it would be a lot of dubious
>> and likely not performing well effort just to save 8 bytes.
>>
>
> The crazy idea from Eric looks easier and more clearly to me than the 
> discussed patch set from Patrick Ohly - but i wonder if we should give a 
> separate hw timestamp a try ...

For reasons that have been mentioned already here (some hardware can
time stamp every packet, new use cases) I think it would be important
to have the hwtstamp information right in the skb. I can change the
patch series so that it uses one additional ktime_t hwtstamp field; give
me a few days, I'm currently traveling.

> I know Patrick is not a friend of a CONFIG option here. But when we make 
> it right HW timestamp could only be disabled on CONFIG_EMBEDDED or 
> something like that.

I'm not a friend of a config option because it was suggested that
hardware tstamps should off on *standard* kernels. That's of little
use for users of unmodified distributions who want to run PTP.

If the feature is only disabled on special distributions, then I really
don't mind, but at the same time I wonder whether these distributions
are performance sensitive enough to care about the additional field.

Bye, Patrick
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