From mboxrd@z Thu Jan 1 00:00:00 1970 From: Miroslav Lichvar Subject: [PATCH v5 net-next 5/7] net: fix documentation of struct scm_timestamping Date: Thu, 18 May 2017 16:07:36 +0200 Message-ID: <20170518140738.19617-6-mlichvar@redhat.com> References: <20170518140738.19617-1-mlichvar@redhat.com> Cc: Richard Cochran , Willem de Bruijn To: netdev@vger.kernel.org Return-path: Received: from mx1.redhat.com ([209.132.183.28]:50905 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756108AbdEROHv (ORCPT ); Thu, 18 May 2017 10:07:51 -0400 In-Reply-To: <20170518140738.19617-1-mlichvar@redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: The scm_timestamping struct may return multiple non-zero fields, e.g. when both software and hardware RX timestamping is enabled, or when the SO_TIMESTAMP(NS) option is combined with SCM_TIMESTAMPING and a false software timestamp is generated in the recvmsg() call in order to always return a SCM_TIMESTAMP(NS) message. CC: Richard Cochran CC: Willem de Bruijn Signed-off-by: Miroslav Lichvar --- Documentation/networking/timestamping.txt | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/Documentation/networking/timestamping.txt b/Documentation/networking/timestamping.txt index 600c6bf..74b7c61 100644 --- a/Documentation/networking/timestamping.txt +++ b/Documentation/networking/timestamping.txt @@ -321,7 +321,7 @@ struct scm_timestamping { }; The structure can return up to three timestamps. This is a legacy -feature. Only one field is non-zero at any time. Most timestamps +feature. At least one field is non-zero at any time. Most timestamps are passed in ts[0]. Hardware timestamps are passed in ts[2]. ts[1] used to hold hardware timestamps converted to system time. @@ -330,6 +330,12 @@ a HW PTP clock source, to allow time conversion in userspace and optionally synchronize system time with a userspace PTP stack such as linuxptp. For the PTP clock API, see Documentation/ptp/ptp.txt. +Note that if the SO_TIMESTAMP or SO_TIMESTAMPNS option is enabled +together with SO_TIMESTAMPING using SOF_TIMESTAMPING_SOFTWARE, a false +software timestamp will be generated in the recvmsg() call and passed +in ts[0] when a real software timestamp is missing. For this reason it +is not recommended to combine SO_TIMESTAMP(NS) with SO_TIMESTAMPING. + 2.1.1 Transmit timestamps with MSG_ERRQUEUE For transmit timestamps the outgoing packet is looped back to the -- 2.9.3