From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Riesch Subject: Re: [PATCH 6/8] ptp: Added a clock that uses the eTSEC found on the MPC85xx. Date: Thu, 23 Sep 2010 23:26:56 +0200 Message-ID: <4C9BC620.3@riesch.at> References: <57b64051c816dc9cb856bbb9f38fc901c9d3d651.1285261535.git.richard.cochran@omicron.at> <20100923214359.3f287b11@lxorguk.ukuu.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20100923214359.3f287b11@lxorguk.ukuu.org.uk> Sender: linux-kernel-owner@vger.kernel.org To: Alan Cox Cc: Christoph Lameter , Richard Cochran , linux-kernel@vger.kernel.org, devicetree-discuss@lists.ozlabs.org, linux-api@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, Arnd Bergmann , David Miller , John Stultz , Krzysztof Halasa , Peter Zijlstra , Rodolfo Giometti , Thomas Gleixner List-Id: linux-api@vger.kernel.org Alan Cox wrote: >> Please do not introduce useless additional layers for clock sync. Load >> these ptp clocks like the other regular clock modules and make them sync >> system time like any other clock. > > I don't think you understand PTP. PTP has masters, a system can need to > be honouring multiple conflicting masters at once. AFAIK the master's should not be conflicting. The Best Master Clock algorithm (BMC) defined in IEEE1588 selects the best master clock. This clock distributes its notion of time on the network while the other masters, that is the other clocks/nodes that are configured to potentially become a master, keep quiet. So usually we will only have one source of time (the master clock selected by the BMC) and we will steer our single PHC (PTP hardware clock) to follow this master (Of course there may be use-cases that require more than one PTP clock, e.g., for research purposes). However, if the clock selected by the BMC is switched off, loses its network connection..., the second best clock is selected by the BMC and becomes master. This clock may be less accurate and thus our slave clock has to switch from one notion of time to another. Is that the conflict you mentioned? Christian