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Mon, 17 Nov 2025 11:45:15 +0000 Date: Mon, 17 Nov 2025 11:45:10 +0000 From: Bruce Richardson To: Jacob Keller CC: , Paul Greenwalt , "Vladimir Medvedkin" Subject: Re: [PATCH 2/2] net/iavf: check Rx timestamp validity bit Message-ID: References: <20251113-jk-dpdk-iavf-rx-timestamping-improvements-v1-0-3d9a0168087b@intel.com> <20251113-jk-dpdk-iavf-rx-timestamping-improvements-v1-2-3d9a0168087b@intel.com> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20251113-jk-dpdk-iavf-rx-timestamping-improvements-v1-2-3d9a0168087b@intel.com> X-ClientProxiedBy: DB8P191CA0012.EURP191.PROD.OUTLOOK.COM (2603:10a6:10:130::22) To DS0PR11MB7309.namprd11.prod.outlook.com (2603:10b6:8:13e::17) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB7309:EE_|LV8PR11MB8607:EE_ X-MS-Office365-Filtering-Correlation-Id: a0d157a5-cf06-48b5-0ff2-08de25cec650 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014; 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In > the event that hardware does not capture a receive timestamp for any > reason, this valid bit is unset, and the timestamp value in the > descriptor is zeroed out. > > The iavf driver ignores this and passes the zero value into the > iavf_tstamp_convert_32b_64b function regardless, and proceeds to treat > the result as a valid timestamp. > > Instead of reporting a zero timestamp which users can clearly interpret > as invalid, the raw 0 value from the descriptor is "extended" to the > 64-bit timestamp. This results in values which are not immediately > obvious as invalid to users: > > timestamp 1760629088881475583 > timestamp 1760629088881475583 > timestamp 1760629088881475583 > > First, if the value is printed in base 10 it is not immediately obvious > that the lower 32 bits are zero. Second, multiple packets in sequence > will receive the same "timestamp". > > This occurs because of the timestamp extension logic. The receive > descriptor timestamps are 40 bits, with 32 bits of nanosecond precision, > 7 bits of subnanosecond precision, and 1 validity bit. The > sub-nanosecond precision bits are discarded. To obtain a 64-bit > timestamp, the upper 32 bits are calculated from the lower 32-bits and a > snapshot of the PHC timer that is captured recently (within ~2 seconds > of the packet timestamp). This enables reporting proper full 64-bit > timestamps without needing to store all 64 bits in the receive > descriptor. > > However, when timestamps are not working properly, the raw 'zero' value > is extended regardless of whether hardware indicated it was a valid > timestamp. As a result, users can see what appear at a glance as valid > timestamps. However, they will not match the packet reception time, and > will only update when the upper bits would roll over. This occurs every > 2^32 seconds, or approximately once every 4 seconds. > > Instead of reporting bogus extended timestamp values which could confuse > user applications, check the validity bit and only report a timestamp of > the valid bit is set. This matches the implementation used in the Linux > PF driver. > > Signed-off-by: Jacob Keller > --- Fixes: b5cd735132f6 ("net/iavf: enable Rx timestamp on flex descriptor") Acked-by: Bruce Richardson