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b=svmnzmKuf1M/QZhWRS3mov4YgG4ialIFHKb0TjWemUc0Mb1y4GgwWhbSsTxe4T+/3UGviUorATWGDY8WctZtkpgbQU3ONBVNTs9TXFJhyPWNjB/Zn2hnKE5SEm4z8hL/a3G3ku8RtBPpAd/SulSAMjsjC1Qfv/yNiLHrAGOt5yQ= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=garyguo.net; Received: from LOVP265MB8871.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:488::16) by CWXP265MB5652.GBRP265.PROD.OUTLOOK.COM (2603:10a6:400:15a::14) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9542.10; Wed, 21 Jan 2026 16:10:01 +0000 Received: from LOVP265MB8871.GBRP265.PROD.OUTLOOK.COM ([fe80::1c3:ceba:21b4:9986]) by LOVP265MB8871.GBRP265.PROD.OUTLOOK.COM ([fe80::1c3:ceba:21b4:9986%5]) with mapi id 15.20.9542.008; Wed, 21 Jan 2026 16:10:01 +0000 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Wed, 21 Jan 2026 16:10:00 +0000 Message-Id: Cc: "Alexandre Courbot" , "lossin@kernel.org" , "a.hindborg@kernel.org" , "boqun.feng@gmail.com" , "aliceryhl@google.com" , "Zhi Wang" , "simona@ffwll.ch" , "alex.gaynor@gmail.com" , "ojeda@kernel.org" , "tmgross@umich.edu" , "nouveau@lists.freedesktop.org" , "linux-kernel@vger.kernel.org" , "rust-for-linux@vger.kernel.org" , "bjorn3_gh@protonmail.com" , "Edwin Peer" , "airlied@gmail.com" , "Joel Fernandes" , "bhelgaas@google.com" , "gary@garyguo.net" , "Alistair Popple" Subject: Re: [PATCH 17/31] gpu: nova-core: Hopper/Blackwell: calculate reserved FB heap size From: "Gary Guo" To: "John Hubbard" , "Timur Tabi" , "dakr@kernel.org" X-Mailer: aerc 0.21.0 References: <20251203055923.1247681-1-jhubbard@nvidia.com> <20251203055923.1247681-18-jhubbard@nvidia.com> <4e36e160bfa53240987f65ba36cae8e6fe4d6e6c.camel@nvidia.com> In-Reply-To: X-ClientProxiedBy: LO4P265CA0198.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:318::11) To LOVP265MB8871.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:488::16) 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: LOVP265MB8871:EE_|CWXP265MB5652:EE_ X-MS-Office365-Filtering-Correlation-Id: 6fd751a5-8d6e-4be1-1734-08de590787bc X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|10070799003|376014|7416014; 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>>> +=C2=A0=C2=A0=C2=A0 (value + (ALIGN - 1)) & !(ALIGN - 1) >>> +} >>=20 >> So this is just like the issue I'm having with .next_multiple_of() in my= patch #10. >>=20 >> Shouldn't you have a check to make sure that value + (ALIGN - 1) doesn't= overflow? Since I need to >> align up to the nearest const power of two, I could use this function in= stead of align_up() and >> avoid testing for an error condition. >>=20 > > Const items are special: they are evaluated at compile time, rather > than at runtime. And so this will fail to compile, for values that > would overflow. > > For example: > > error[E0080]: evaluation of constant value failed > --> drivers/gpu/nova-core/num.rs:226:5 > | > 226 | (value + (ALIGN - 1)) & !(ALIGN - 1) > | ^^^^^^^^^^^^^^^^^^^^^ attempt to compute `usize::MAX + 15_usiz= e`, which would overflow > | > note: inside `const_align_up::<16>` > --> drivers/gpu/nova-core/num.rs:226:5 > | > 226 | (value + (ALIGN - 1)) & !(ALIGN - 1) > | ^^^^^^^^^^^^^^^^^^^^^ > note: inside `_OVERFLOW_TEST` > --> drivers/gpu/nova-core/num.rs:231:31 > | > 231 | const _OVERFLOW_TEST: usize =3D const_align_up::<16>(usize::MAX); > | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Note that this is only true if the function is only ever called from compil= e time. If the const function is indeeed used during runtime, you'll lose the= se checks (or, depending on Kconfig, get turned into BUG). Best, Gary > > > thanks,