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Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Subject: Re: [PATCH V3] Revert "PCI: tegra194: Enable support for 256 Byte payload" To: Bjorn Helgaas Cc: lpieralisi@kernel.org, kw@linux.com, robh@kernel.org, bhelgaas@google.com, thierry.reding@gmail.com, jonathanh@nvidia.com, Sergey.Semin@baikalelectronics.ru, linux-pci@vger.kernel.org, linux-tegra@vger.kernel.org, linux-kernel@vger.kernel.org, kthota@nvidia.com, mmaddireddy@nvidia.com, sagar.tv@gmail.com References: <20230718110913.GA475601@bhelgaas> Content-Language: en-US From: Vidya Sagar In-Reply-To: <20230718110913.GA475601@bhelgaas> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MA1PR01CA0150.INDPRD01.PROD.OUTLOOK.COM (2603:1096:a00:71::20) To BN8PR12MB2900.namprd12.prod.outlook.com (2603:10b6:408:69::18) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BN8PR12MB2900:EE_|MN0PR12MB5739:EE_ X-MS-Office365-Filtering-Correlation-Id: f9dd335d-2f40-4e9e-870e-08db88477f27 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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In this scenario, the default bus >>>> configuration of the kernel i.e. "PCIE_BUS_DEFAULT" doesn't configure the >>>> MPS settings in the hierarchy correctly resulting in the device with >>>> support for minimum MPS in the hierarchy receiving the TLPs of size more >>>> than that. Although this can be addressed by appending "pci=pcie_bus_safe" >>>> to the kernel command line, it doesn't seem to be a good idea to always >>>> have this commandline argument even for the basic functionality to work. >>> >>> I think this has some irrelevant detail (IIUC the problem should >>> happen even without a switch) and could be more specific (I think the >>> problem case is RP MPS=256, EP only supports MPS=128). >> >> The issue is present only if there is a switch. > > So if there's no switch, and an EP that only supports MPS=128, the PCI > core changes the RP MPS setting to 128? Just based on reading the Yes. The code after the if condition here takes care of that. https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/probe.c?h=v6.5-rc2#n2049 > code, I thought we would leave RP MPS=256 and EP MPS=128, which would > be a problem. But maybe the PCI core changes the RP down to MPS=128? > > Bjorn