From mboxrd@z Thu Jan 1 00:00:00 1970 From: Patrick Ohly Subject: Re: hardware time stamps + existing time stamp usage Date: Tue, 21 Oct 2008 09:40:39 +0200 Message-ID: <1224574839.17450.332.camel@ecld0pohly> References: <1224253423.17450.211.camel@ecld0pohly> <200810182237.02373.opurdila@ixiacom.com> <48FD7EF5.6050805@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: Octavian Purdila , "netdev@vger.kernel.org" , Stephen Hemminger , Ingo Oeser , "Ronciak, John" To: Andi Kleen Return-path: Received: from mga09.intel.com ([134.134.136.24]:26805 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751412AbYJUHmv (ORCPT ); Tue, 21 Oct 2008 03:42:51 -0400 In-Reply-To: <48FD7EF5.6050805@linux.intel.com> Sender: netdev-owner@vger.kernel.org List-ID: On Tue, 2008-10-21 at 01:04 -0600, Andi Kleen wrote: > > We can even compute the delta periodically now, to maintain better system - > > hardware timestamps synchronization, as we can keep and multiple deltas (each > > one associated with a modulo number). > > The problem with this scheme is that it's unlikely to be precise enough to guarantee > monoticity (that is that your delta clock compared to the system clock never goes > backwards). And that tends to be a common requirements in system time stamps. > Not having that would risk breaking existing applications. Agreed. But even those users who need absolute monoticity would be able to use PTP: at least the Intel hardware would be configured to only time stamp PTP packets while the application packets that the user cares about are still time stamped in software, as before. > My recommendation would be to find some way to use a separate field and also > use a separate API. That would also allow you to extend it (e.g. pass down > the interface number), so that different time stamps from different interfaces > are supported. The latest proposal already uses such a separate API for HW time stamps, so we are fine in that regard. In my opinion the API should only return information which isn't available otherwise (currently the original HW time stamp); the interface number should be returned with the existing IP_PKTINFO. -- Best Regards, Patrick Ohly The content of this message is my personal opinion only and although I am an employee of Intel, the statements I make here in no way represent Intel's position on the issue, nor am I authorized to speak on behalf of Intel on this matter.