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Mon, 29 Jul 2024 18:08:17 +0000 Message-ID: Date: Mon, 29 Jul 2024 11:08:01 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next v17 04/14] net: Change the API of PHY default timestamp to MAC To: Kory Maincent CC: Florian Fainelli , "Broadcom internal kernel review list" , Andrew Lunn , Heiner Kallweit , Russell King , "David S. Miller" , "Eric Dumazet" , Jakub Kicinski , Paolo Abeni , Richard Cochran , Radu Pirea , Jay Vosburgh , "Andy Gospodarek" , Nicolas Ferre , Claudiu Beznea , Willem de Bruijn , Jonathan Corbet , "Horatiu Vultur" , , "Simon Horman" , Vladimir Oltean , , , , Thomas Petazzoni , , , , Maxime Chevallier , Rahul Rameshbabu , Willem de Bruijn , Shannon Nelson , Alexandra Winter References: <20240709-feature_ptp_netnext-v17-0-b5317f50df2a@bootlin.com> <20240709-feature_ptp_netnext-v17-4-b5317f50df2a@bootlin.com> <39c7fe45-fbee-4de5-ab43-bf042ed31504@intel.com> <20240727154426.7ba30ed9@kmaincent-XPS-13-7390> Content-Language: en-US From: Jacob Keller In-Reply-To: <20240727154426.7ba30ed9@kmaincent-XPS-13-7390> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4PR04CA0116.namprd04.prod.outlook.com (2603:10b6:303:83::31) To CO1PR11MB5089.namprd11.prod.outlook.com (2603:10b6:303:9b::16) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO1PR11MB5089:EE_|DS0PR11MB7384:EE_ X-MS-Office365-Filtering-Correlation-Id: 84c63e5d-665c-48d9-a5c1-08dcaff96c34 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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Due to lower >>> time stamping clock frequency, latency in the MDIO bus and no PHC hardware >>> synchronization between different PHY, the PHY PTP is often less precise >>> than the MAC. The exception is for PHY designed specially for PTP case but >>> these devices are not very widespread. For not breaking the compatibility >>> default_timestamp flag has been introduced in phy_device that is set by >>> the phy driver to know we are using the old API behavior. >>> >> >> This description feels like it is making a pretty broad generalization >> about devices. The specifics of whether MAC or PHY timestamping is >> better will be device dependent. > > As explained, except for specific PTP specialized PHY, the MAC is better in > term of PTP precision. > This patch was a requisite from Russell, who wanted to add support for the PTP > in the marvell PHY. Doing so would select the PHY PTP by default which cause a > regression as the PHY hardware timestamp is less precise than the MAC. > https://lore.kernel.org/netdev/20200729105807.GZ1551@shell.armlinux.org.uk/ > https://lore.kernel.org/netdev/Y%2F4DZIDm1d74MuFJ@shell.armlinux.org.uk/ > There is also discussion on how to support it in older version of this series. > Right. So it is a bit of a generalization, but in practice it matches up with the available hardware on the market. >> It looks like you introduce a default_timestamp flag to ensure existing >> devices default to PHY? I assume your goal here is to discourage this >> and not allow setting it for new devices? Or do we want to let device >> driver authors decide which is a better default? > > Yes to not change the old behavior the current PHY with PTP support will still > behave as default PTP. The point is indeed to discourage future drivers to > select the PHY as default PTP. > Ok great! >>> diff --git a/net/core/timestamping.c b/net/core/timestamping.c >>> index 04840697fe79..3717fb152ecc 100644 >>> --- a/net/core/timestamping.c >>> +++ b/net/core/timestamping.c >>> @@ -25,7 +25,8 @@ void skb_clone_tx_timestamp(struct sk_buff *skb) >>> struct sk_buff *clone; >>> unsigned int type; >>> >>> - if (!skb->sk) >>> + if (!skb->sk || !skb->dev || >>> + !phy_is_default_hwtstamp(skb->dev->phydev)) >> >> I don't follow why this check is added and its not calling something >> like "phy_is_current_hwtstamp"? I guess because we don't yet have a way >> to select between MAC/PHY at this point in the series? Ok. > > skb_clone_tx_timestamp is only used for PHY timestamping so we should do nothing > if the default PTP is the MAC. > I guess my misunderstanding is what about the case where user selects PHY timestamping with the netlink command? Then it would still need to do the skb_clone_tx_timestamp even though its not the default? Or does phy_is_default_hwtstamp take that into account? In which case it would make more sense to name it phy_is_current_hwtstamp. Either way this is mostly bikeshedding and probably just some misunderstanding in my reading of the code. Thanks, Jake > Regards,