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Thu, 3 Apr 2025 04:42:58 +0000 Date: Wed, 2 Apr 2025 21:42:55 -0700 From: Dan Williams To: "Zhijian Li (Fujitsu)" , Gregory Price , "linux-cxl@vger.kernel.org" CC: "linux-kernel@vger.kernel.org" , "kernel-team@meta.com" , "dan.j.williams@intel.com" , "vishal.l.verma@intel.com" , "dave.jiang@intel.com" , "dave@stgolabs.net" , "jonathan.cameron@huawei.com" , "alison.schofield@intel.com" , "ira.weiny@intel.com" , "Yasunori Gotou (Fujitsu)" Subject: Re: [PATCH v2] cxl: core/region - ignore interleave granularity when ways=1 Message-ID: <67ee11cf28f2e_464ec2949a@dwillia2-xfh.jf.intel.com.notmuch> References: <20250402232552.999634-1-gourry@gourry.net> <2ce69e60-8a13-4e9f-b28f-1b30162a1ada@fujitsu.com> Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <2ce69e60-8a13-4e9f-b28f-1b30162a1ada@fujitsu.com> X-ClientProxiedBy: MW4PR03CA0061.namprd03.prod.outlook.com (2603:10b6:303:b6::6) To PH8PR11MB8107.namprd11.prod.outlook.com (2603:10b6:510:256::6) Precedence: bulk X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR11MB8107:EE_|PH7PR11MB6723:EE_ X-MS-Office365-Filtering-Correlation-Id: 38dc02e0-f366-4ad5-5e28-08dd726a023a X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014; 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Yes, the value does not matter, but attempting 1024 feels more like a unit test than a production use case. > > With this setup, the Linux kernel attempts to create regions for Endpoint9 and Endpoint10 > but fails because the endpoint decoders’ interleave granularity (IG=1024) does not > match their parent decoders’ IG (256). Ideally, the endpoint decoders are expected to be > configured for IG=256. > > Currently, we learned that we have only special handling for the root decoders [2][3]. > > My question are: > Q1: whether "granularity is a don't care if not interleaving" is applied to > all HDM decoders(including root decoder and HDM decoder) All decoders. > In current cxl cli , it will not show any interleave_granularity at all when ways==1(no-interleaving) > $ cxl list -PDE | grep granularity # show nothing when ways==1 Right, because the value theoretically has no functional impact in the ways==1 case. However, it errantly ends up having practical impact in these corners cases where code performs granularity comparisons without considering that ways may be 1. > Per the CXL Spec r3.1 > IG: "The number of consecutive bytes that are assigned to each target in the Target List." > Q2: Does this imply a configuration where the number of ways>1? Right, the granularity is the boundary at which the decoder switches to the next target in the target list. When ways=1 granularity can be infinity or zero by that definition. > Q3: Does the IG also represent the device's capabilities? When programming, should one also > consider whether the device supports it? Yes, see bits [9:8] in the CXL HDM Decoder Capability Register (CXL 3.2 8.2.4.20.1). So even though the math should not matter, I would still expect the driver to try to be careful to make sure that IG+8 is less than the address-bit max. > If "granularity is a don't care if not interleaving" is true, how about below changes Part of me says, "yes, that should be ok", another part of me says "what is the practical benefit of allowing any granularity to be specified?". So the fix from Gregory is limited to the case of "whoops, the platform BIOS thought this was a good idea even though it does not matter in practice, teach Linux to be lenient in this case.". The proposal to accept that in all case allows user-created regions to have odd large granularity sizes in the iw=1 case, and I am skeptical it is worth supporting that now.