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Miller" , Saeed Mahameed , Leon Romanovsky , Mark Bloch , Richard Cochran , netdev@vger.kernel.org, linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org References: <1752556533-39218-1-git-send-email-tariqt@nvidia.com> <20250721170916.490ce57e@kernel.org> <0c1cea33-6676-4590-8c7c-9fe1a3d88f0b@nvidia.com> <20250729154012.5d540144@kernel.org> Content-Language: en-US From: Carolina Jubran In-Reply-To: <20250729154012.5d540144@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: TLZP290CA0013.ISRP290.PROD.OUTLOOK.COM (2603:1096:950:9::6) To MW4PR12MB7141.namprd12.prod.outlook.com (2603:10b6:303:213::20) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: MW4PR12MB7141:EE_|DS7PR12MB6357:EE_ X-MS-Office365-Filtering-Correlation-Id: 3206fb6b-7e10-4444-be15-08ddd064e611 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014|7416014; 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User space can periodically sample the >> (cycle, time) pairs returned by the new ioctl to estimate the clock’s >> frequency and detect anomalies, for example, drift caused by temperature >> changes. This is especially useful in holdover scenarios. > > Because the servo running on the host doesn't know the stability? > Seems like your real use case is the one below. > >> Another practical case is with DPDK. When the hardware is in FreeRunning >> mode, the CQE contains raw cycle counter values. DPDK returns these >> values directly to user space without converting them. The new ioctl >> provides a generic and consistent way to translate those raw values to >> host time. >> >> As for XDP, you’re right that it doesn’t expose raw cycles today. The >> point here is more future-looking: if drivers ever choose to emit raw >> cycles into metadata for performance, this API gives user space a clean >> way to interpret those timestamps. > > Got it, I can see how DPDK / kernel bypass may need this. > > Please include this justification in the commit message for v2 > and let's see if anyone merges it. Thanks, I’ll include the DPDK/kernel bypass justification clearly in the v2 commit message and cover letter. Additionally, I wanted to mention another relevant use case that wasn’t brought up earlier: fwctl can expose event records tagged with raw cycle counter timestamps. When the device is in free-running mode, correlating those with host time becomes difficult unless user space has access to both cycle and system time snapshots.