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Mon, 23 Jan 2023 18:14:06 +0000 Received: from CO1PR11MB5089.namprd11.prod.outlook.com ([fe80::5697:a11e:691e:6acf]) by CO1PR11MB5089.namprd11.prod.outlook.com ([fe80::5697:a11e:691e:6acf%5]) with mapi id 15.20.6002.033; Mon, 23 Jan 2023 18:14:06 +0000 Message-ID: <71c3e9a0-d14f-4748-dfe6-6a41b323818f@intel.com> Date: Mon, 23 Jan 2023 10:14:03 -0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.6.1 Subject: Re: [net-next 03/15] net/mlx5: Add adjphase function to support hardware-only offset control Content-Language: en-US To: Richard Cochran CC: Rahul Rameshbabu , Jakub Kicinski , Saeed Mahameed , "David S. Miller" , Paolo Abeni , Eric Dumazet , Saeed Mahameed , , Tariq Toukan , Gal Pressman , Vincent Cheng References: <20230118183602.124323-1-saeed@kernel.org> <20230118183602.124323-4-saeed@kernel.org> <739b308c-33ec-1886-5e9d-6c5059370d15@intel.com> <20230119194631.1b9fef95@kernel.org> <87tu0luadz.fsf@nvidia.com> <20230119200343.2eb82899@kernel.org> <87pmb9u90j.fsf@nvidia.com> <8da8ed6a-af78-a797-135d-1da2d5a08ca1@intel.com> <87r0vpcch0.fsf@nvidia.com> <3312dd93-398d-f891-1170-5d471b3d7482@intel.com> From: Jacob Keller In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SJ0PR13CA0183.namprd13.prod.outlook.com (2603:10b6:a03:2c3::8) To CO1PR11MB5089.namprd11.prod.outlook.com (2603:10b6:303:9b::16) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO1PR11MB5089:EE_|PH7PR11MB7098:EE_ X-MS-Office365-Filtering-Correlation-Id: 25c7803e-6c94-4be2-776d-08dafd6d9d4c X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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I guess what I don't understand is why or when one would want to >> use adjphase instead of just performing frequency adjustment via the >> .adjfine operation... > > The difference is in where the frequency adjust is calculated. There > are two possibilities: > > 1. It may be done in user space. > This is NTP timex's ADJ_FREQUENCY. > PHC implements .adjfine > > 2. It may be left to kernel space. > This is NTP timex's ADJ_OFFSET. > PHC implements .adjphase > > In case of #2, the hardware implements some kind of clock servo. The > inputs to the servo are the reported phase offsets. The output of the > servo is a frequency adjustment. > >> Especially since "gradual adjustment over time" means it will still be >> slow to converge (just like adjusting frequency is today). > > It depends on the servo parameters. > >> We should definitely improve the doc to explain the diff between them >> and make sure that its more clear to driver implementations. > > Sure. > >> It also makes it harder to justify mapping small .adjtime to .adjphase, > > adjtime is totally different. Don't mix those two up: > > - adjtime sets the time (phase), plain and simple. > > - adjphase feeds the phase offsets into a servo algorithm in hardware. > > FWIW, the PHC subsystem handles the semantics of timex ADJ_FREQUENCY > and ADJ_OFFSET in exactly the same way as the NTP subsystem. > > > Thanks, > Richard Thanks for a succinct summary!