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Fri, 12 Dec 2025 07:57:51 +0000 Message-ID: <684b2ce6-c8f4-4ecf-9fbc-b75137689083@intel.com> Date: Fri, 12 Dec 2025 08:57:42 +0100 User-Agent: Mozilla Thunderbird Subject: Re: [Intel-wired-lan] [PATCH net v1] idpf: read lower clock bits inside the time sandwich To: Jacob Keller , "Loktionov, Aleksandr" , Mina Almasry CC: "Nguyen, Anthony L" , Andrew Lunn , "David S. Miller" , "netdev@vger.kernel.org" , Eric Dumazet , "linux-kernel@vger.kernel.org" , Jakub Kicinski , Paolo Abeni , Richard Cochran , "Rizzo, Luigi" , "namangulati@google.com" , "willemb@google.com" , "intel-wired-lan@lists.osuosl.org" , "Olech, Milena" , Shachar Raindel References: <20251211101931.2788437-1-almasrymina@google.com> <25a83e83-46d1-4a16-9383-6d492cb37c7a@intel.com> From: Przemek Kitszel Content-Language: en-US In-Reply-To: <25a83e83-46d1-4a16-9383-6d492cb37c7a@intel.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: DU7P194CA0012.EURP194.PROD.OUTLOOK.COM (2603:10a6:10:553::18) To MN6PR11MB8102.namprd11.prod.outlook.com (2603:10b6:208:46d::9) 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: CH0PR11MB8086:EE_|DS0PR11MB7507:EE_ X-MS-Office365-Filtering-Correlation-Id: d9180e09-2dc8-4135-ddfb-08de39542426 X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|366016|1800799024|7053199007; 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linux-kernel@vger.kernel.org >>> Cc: Mina Almasry ; Nguyen, Anthony L >>> ; Kitszel, Przemyslaw >>> ; Andrew Lunn ; >>> David S. Miller ; Eric Dumazet >>> ; Jakub Kicinski ; Paolo Abeni >>> ; Richard Cochran ; >>> Rizzo, Luigi ; namangulati@google.com; >>> willemb@google.com; intel-wired-lan@lists.osuosl.org; Olech, Milena >>> ; Keller, Jacob E ; >>> Shachar Raindel >>> Subject: [Intel-wired-lan] [PATCH net v1] idpf: read lower clock bits >>> inside the time sandwich >>> >>> PCIe reads need to be done inside the time sandwich because PCIe >>> writes may get buffered in the PCIe fabric and posted to the device >>> after the _postts completes. Doing the PCIe read inside the time >>> sandwich guarantees that the write gets flushed before the _postts >>> timestamp is taken. >>> >>> Cc: lrizzo@google.com >>> Cc: namangulati@google.com >>> Cc: willemb@google.com >>> Cc: intel-wired-lan@lists.osuosl.org >>> Cc: milena.olech@intel.com >>> Cc: jacob.e.keller@intel.com >>> >>> Fixes: 5cb8805d2366 ("idpf: negotiate PTP capabilities and get PTP >>> clock") >>> Suggested-by: Shachar Raindel >>> Signed-off-by: Mina Almasry >>> --- >>> drivers/net/ethernet/intel/idpf/idpf_ptp.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/drivers/net/ethernet/intel/idpf/idpf_ptp.c >>> b/drivers/net/ethernet/intel/idpf/idpf_ptp.c >>> index 3e1052d070cf..0a8b50350b86 100644 >>> --- a/drivers/net/ethernet/intel/idpf/idpf_ptp.c >>> +++ b/drivers/net/ethernet/intel/idpf/idpf_ptp.c >>> @@ -108,11 +108,11 @@ static u64 >>> idpf_ptp_read_src_clk_reg_direct(struct idpf_adapter *adapter, >>> ptp_read_system_prets(sts); >>> >>> idpf_ptp_enable_shtime(adapter); >>> + lo = readl(ptp->dev_clk_regs.dev_clk_ns_l); >> The high 32 bits (hi) are still read outside the time sandwich (after ptp_read_system_postts()), >> which defeats the stated purpose of ensuring PCIe write flush before timestamp capture. >> /* I think he "time sandwich" is defined by the region between ptp_read_system_prets(sts) and ptp_read_system_postts(sts) */ Isn't it? >> >> > > Any read will cause writes to flush, so we don't need to move both > registers. > > The point here is that we write to the shadow register to snapshot time, > and it won't guarantee to be flushed to the device until a read. By > moving a single read in side the time sandwhich, we ensure that its > actually complete before the time snapshot is taken. We don't need to > wait for both registers because of the snapshot behavior. very nice explanation Jake, thank you I don't know if it should be considered "basic common knowledge", or warrants an entry in commit message/code comment For sure we don't want anyone not knowing that to touch the code, so barrier to entry is also a good thing ;) > > I think the patch is fine-as-is. given the scope of the function, agree > > Reviewed-by: Jacob Keller > > Thanks, > Jake