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charset=UTF-8 Date: Thu, 25 Jun 2026 09:45:53 +0900 Message-Id: Cc: "David Laight" , "Zhi Wang" , , , , , , , , , , , , , , , , , , , , , , , , , , , "Bjorn Helgaas" , Subject: Re: [PATCH v2 1/7] PCI/IOV: Return u16 from pci_sriov_get_totalvfs() From: "Alexandre Courbot" To: "Gary Guo" References: <20260622194353.1308872-1-zhiw@nvidia.com> <20260622194353.1308872-2-zhiw@nvidia.com> <20260624143937.50499c29@pumpkin> <20260624203851.7f6c7be4@pumpkin> In-Reply-To: X-ClientProxiedBy: TY6PR01CA0023.jpnprd01.prod.outlook.com (2603:1096:405:3bb::6) 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_|IA1PR12MB6233:EE_ X-MS-Office365-Filtering-Correlation-Id: b1d89bf4-da7b-437b-ac02-08ded2531f36 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|23010399003|7416014|376014|10070799003|1800799024|18002099003|22082099003|6133799003|11063799006|5023799004|4143699003|56012099006; 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It returns 0 when SR-IOV is unavailable or the device is n= ot a >>> >> > PF, and otherwise returns the PF's driver_max_VFs value. >>> >> > >>> >> > driver_max_VFs is stored as a u16 in struct pci_sriov. It is deriv= ed >>> >> > from the SR-IOV TotalVFs field or from a driver-provided limit, so= the >>> >> > implementation cannot return a negative value. >>> >> > >>> >> > Change the declaration, CONFIG_PCI_IOV stub, and implementation to >>> >> > return u16. Update callers to store the result in u16 variables, r= emove >>> >> > obsolete negative-value checks, and use unsigned format specifiers= where >>> >> > needed. >>> >> > >>> >> > Cc: Bjorn Helgaas >>> >> > Cc: linux-pci@vger.kernel.org >>> >> > Signed-off-by: Zhi Wang =20 >>> >>=20 >>> >> Suggested-by: Alexandre Courbot >>> >> Link: https://lore.kernel.org/all/DETDILPA1GFY.27WND0TEC5352@nvidia.= com/ >>> >> =20 >>> >> > --- >>> >> > drivers/crypto/hisilicon/qm.c | 8 +++++--- >>> >> > drivers/crypto/intel/qat/qat_common/adf_sriov.c | 6 +++--- >>> >> > drivers/gpu/drm/xe/xe_sriov_pf.c | 6 ++---- >>> >> > drivers/misc/genwqe/card_base.c | 6 ++---- >>> >> > drivers/net/ethernet/cavium/thunder/nic_main.c | 2 +- >>> >> > drivers/net/ethernet/emulex/benet/be_main.c | 3 ++- >>> >> > drivers/net/ethernet/mellanox/mlx5/core/sriov.c | 3 ++- >>> >> > drivers/net/ethernet/sfc/ef10_sriov.c | 2 +- =20 >>> >>=20 >>> >> I believe that you can avoid converting all these drivers in this pa= tch. >>> >> The implicit `u16 -> int` conversion done by C should result in the >>> >> expected behavior, and it will be fewer Acked-by to collect. =20 >>> > >>> > The generated code is also likely to be slightly better if the functi= on >>> > return value is a 32bit value. >>> > >>> > Similarly you don't really want to do any kind of maths on local vari= ables >>> > that aren't 32bit (or 64bit on 64bit builds). >>> > >>> > The fact that the domain of a value fits in 16 bits doesn't mean that >>> > it is better to use u16 - it is usually worse. >>> > Pretty much the only place u16 should be used is to reduce the size >>> > of structures. >>> > >>> > So it is probably correct to change the return type to unsigned int a= nd >>> > remove the error return checks, but nothing else. =20 >>>=20 >>> For C, I agree that unsigned int is the safest type. >>>=20 >>> Rust otoh does not do implicit integer promotion, and making it return = a >>> `u16` carries useful range information. I wonder if we could have a >>> private `__pci_sriov_get_totalvfs` that returns a `u16`, make >>> `pci_sriov_get_totalvfs` promote it to an `unsigned int` and return it, >>> while the Rust bindings would invoke `__pci_sriov_get_totalvfs` so they >>> can expose a `u16`? > > There're no value in having two versions with just slightly different pro= motion > behaviour. Rust side can still use u16 by casting from unsigned int witho= ut > having to expose both. Indeed, that occurred to me only after I sent my reply. Since the register this value originates from is 16 bits, we can justify that a cast of the return to u16 won't lose any data. Just making the C API return unsigned int is fine then.