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charset=UTF-8 Date: Fri, 27 Feb 2026 21:00:53 +0900 Message-Id: Cc: , "Miguel Ojeda" , "Boqun Feng" , "Gary Guo" , "Bjorn Roy Baron" , "Benno Lossin" , "Andreas Hindborg" , "Alice Ryhl" , "Trevor Gross" , "Danilo Krummrich" , "Dave Airlie" , "Daniel Almeida" , "Koen Koning" , , , , "Nikola Djukic" , "Maarten Lankhorst" , "Maxime Ripard" , "Simona Vetter" , "Jonathan Corbet" , "Alex Deucher" , "Christian Koenig" , "Jani Nikula" , "Joonas Lahtinen" , "Rodrigo Vivi" , "Tvrtko Ursulin" , "Huang Rui" , "Matthew Auld" , "Matthew Brost" , "Lucas De Marchi" , "Thomas Hellstrom" , "Helge Deller" , "Alex Gaynor" , "Boqun Feng" , "Alistair Popple" , "Andrea Righi" , "Zhi Wang" , "Philipp Stanner" , "Elle Rhumsaa" , , "Eliot Courtney" , , , , , Subject: Re: [PATCH v11 4/4] rust: gpu: Add GPU buddy allocator bindings From: "Alexandre Courbot" To: "Joel Fernandes" References: <20260224224005.3232841-1-joelagnelf@nvidia.com> <20260224224005.3232841-5-joelagnelf@nvidia.com> <3161a017-a9f8-465c-b4dd-fef085d75b98@nvidia.com> In-Reply-To: <3161a017-a9f8-465c-b4dd-fef085d75b98@nvidia.com> X-ClientProxiedBy: TY4P286CA0136.JPNP286.PROD.OUTLOOK.COM (2603:1096:405:37f::13) To CH2PR12MB3990.namprd12.prod.outlook.com (2603:10b6:610:28::18) 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: CH2PR12MB3990:EE_|PH8PR12MB7028:EE_ X-MS-Office365-Filtering-Correlation-Id: 544f301f-e9fe-4ead-a23f-08de75f7ddcc X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|10070799003|376014|7416014|366016|1800799024; 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>> let flags =3D mode.into_flags() | u32::from(flags) as usize; >> // ... and just invoke the C API with these parameters. >>=20 >>> if the C allocator evolves its flag semantics (new combinations become >>> valid, or existing constraints change), an enum on the Rust side would >>> break. It's simpler and more maintainable to pass combinable flags and >>> let the C allocator validate -- which it already does. The switch to >>> `impl_flags!` will work for us without over-constraining. >>=20 >> The evolution you describe is speculative and unlikely to happen as it >> would break all C users just the same. And if the C API adds new flags >> or allocation modes, we will have to update the Rust abstraction either >> way. > > How/why would it break C users? Currently top down + range is silently ig= nored, > implementing it is unlikely to break them. > > I also wouldn't call it speculative: top-down within a range is a natural > feature the C allocator could add right? By modeling modes as a mutually > exclusive enum, we're disallowing a flag combination that could become > valid in the future. That's fine for now, but something to keep in mind a= s we > choose this design. We could add a new RangeTopDown mode variant in the f= uture, > though. That said, I've made the switch to the enum as > you suggested since it is cleaner code! And is more Rust-like as you poin= ted. Ah, I thought you were talking about new flags - the case you are quoting is different, and indeed more concerning. I guess if things were to change in that direction we would have to move top-down into its own flag and update our users. But for now we should try to support the existing actual semantics as closely as possible, and without any surprise if possible - what you get is exactly what you get, without anything swept under the rug (the silent masking done on the C side freaks me out a little bit tbh ^_^;). Should top-down become a flag, our current API also wouldn't become incorrect - it would just be incomplete in that it doesn't support the new use-case until we update it, so thankfully that wouldn't be a critical issue.