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Wed, 18 Dec 2024 15:08:16 +0000 Subject: Re: [Intel-wired-lan] [PATCH iwl-next v3] e1000e: Fix real-time violations on link up To: Gerhard Engleder , , , CC: , , , , , , , , , Gerhard Engleder , Vitaly Lifshits References: <20241214191623.7256-1-gerhard@engleder-embedded.com> From: Avigail Dahan Message-ID: Date: Wed, 18 Dec 2024 17:08:09 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 In-Reply-To: <20241214191623.7256-1-gerhard@engleder-embedded.com> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: TL0P290CA0015.ISRP290.PROD.OUTLOOK.COM (2603:1096:950:5::7) To BY5PR11MB4194.namprd11.prod.outlook.com (2603:10b6:a03:1c0::13) Precedence: bulk X-Mailing-List: linux-pci@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BY5PR11MB4194:EE_|CY5PR11MB6139:EE_ X-MS-Office365-Filtering-Correlation-Id: ea60d329-e22e-4e3f-fdb4-08dd1f75ccf2 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|1800799024|376014|7416014|366016|7053199007; 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This update executes many > PCIe writes and a final flush. Thus, PCIe will be blocked until all > writes are flushed. As a result, DMA transfers of other targets suffer > from delay in the range of 50us. This results in timing violations on > real-time systems during link down and up of e1000e in combination with > an Intel i3-2310E Sandy Bridge CPU. > > The i3-2310E is quite old. Launched 2011 by Intel but still in use as > robot controller. The exact root cause of the problem is unclear and > this situation won't change as Intel support for this CPU has ended > years ago. Our experience is that the number of posted PCIe writes needs > to be limited at least for real-time systems. With posted PCIe writes a > much higher throughput can be generated than with PCIe reads which > cannot be posted. Thus, the load on the interconnect is much higher. > Additionally, a PCIe read waits until all posted PCIe writes are done. > Therefore, the PCIe read can block the CPU for much more than 10us if a > lot of PCIe writes were posted before. Both issues are the reason why we > are limiting the number of posted PCIe writes in row in general for our > real-time systems, not only for this driver. > > A flush after a low enough number of posted PCIe writes eliminates the > delay but also increases the time needed for MTA table update. The > following measurements were done on i3-2310E with e1000e for 128 MTA > table entries: > > Single flush after all writes: 106us > Flush after every write: 429us > Flush after every 2nd write: 266us > Flush after every 4th write: 180us > Flush after every 8th write: 141us > Flush after every 16th write: 121us > > A flush after every 8th write delays the link up by 35us and the > negative impact to DMA transfers of other targets is still tolerable. > > Execute a flush after every 8th write. This prevents overloading the > interconnect with posted writes. > > Reviewed-by: Przemek Kitszel > CC: Vitaly Lifshits > Link: https://lore.kernel.org/netdev/f8fe665a-5e6c-4f95-b47a-2f3281aa0e6c@lunn.ch/T/ > Signed-off-by: Gerhard Engleder > --- > v3: > - mention problematic platform explicitly (Bjorn Helgaas) > - improve comment (Paul Menzel) > > v2: > - remove PREEMPT_RT dependency (Andrew Lunn, Przemek Kitszel) > --- > drivers/net/ethernet/intel/e1000e/mac.c | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > Tested-by: Avigail Dahan