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Wed, 30 Sep 2026 11:24:26 -0700 Received: from nvidia.com (10.127.8.11) by mail.nvidia.com (10.129.68.7) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.49 via Frontend Transport; Wed, 30 Sep 2026 11:24:25 -0700 Date: Wed, 30 Sep 2026 11:24:21 -0700 From: Nicolin Chen To: Pranjal Shrivastava CC: , Will Deacon , Joerg Roedel , Robin Murphy , Jason Gunthorpe , Mostafa Saleh , Daniel Mentz , Ashish Mhetre , , Thomas Gleixner , Radu Rendec , Bjorn Helgaas , , , "Greg Kroah-Hartman" , , "Danilo Krummrich" , Subject: Re: [PATCH v11 03/16] iommu/arm-smmu-v3: Add arm_smmu_drain_queue() helper Message-ID: References: <20260929034510.2023173-1-praan@google.com> <20260929034510.2023173-4-praan@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20260929034510.2023173-4-praan@google.com> X-NV-OnPremToCloud: ExternallySecured X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ1PEPF000037A8:EE_|IA1PR12MB8360:EE_ X-MS-Office365-Filtering-Correlation-Id: 0aef1bd5-7cf3-406e-9895-08df1f201e39 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|23010399003|82310400026|36860700016|7416014|376014|22082099003|18002099003|10067099003|3023799007|4143699003|11063799006|56012099006; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: TAJ2e/C75Ld6Rg7FBawbGL29QgqTJqjdJq8ZSzcYWh9VLIUkAGCrc7IdlRgK74T/i+4ongNymphyFnN8rJMInVl4gnn38L9Gopk8TdCW9s273NHkORFc/u5X7jUYyDcBk1bblz8v23CD1lk7e0UG4qMyusRqBnRrtk833OFNMhuBJ9HcotS9/KrjNnhiGemw0Mj+rWV7zJmt8jjOFzJQU93zzuC9/LyS4rron3iN3u2UfBaTRoKaye28V3z4t0m/MIeULiBh4vhIO3C6CuBjnynQtBPHclMP8GS3rklgk+9xR9npE8JlX9eUJrUTXLnzIvQ9uB9PAdAISV/ditXT2NM9Jf9C6bUqqQPIIqPVzCpcSkwMGF4nzfaZ7ZfNZHgx+acZalKtqKfYrWnKmiy6nPjxjgLgioRhHm1XmUi1OYOuV/sLBMwzlLjiPqF8wq8/ X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 30 Sep 2026 18:24:51.6012 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 0aef1bd5-7cf3-406e-9895-08df1f201e39 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.117.161];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: SJ1PEPF000037A8.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA1PR12MB8360 On Tue, Sep 29, 2026 at 03:44:57AM +0000, Pranjal Shrivastava wrote: > +/** > + * arm_smmu_drain_queue - Drain an SMMU queue > + * @smmu: the SMMU device > + * @q: the queue to drain > + * @until_empty: target selection > + * > + * With @until_empty == true (for CMDQ), exit once the queue is observed empty: > + * > + * cons0 cons prod > + * | | | > + * ---+###################+=====================+=============+---> > + * |<--------- undrained==0? --------->| Jason gave some remarks against this API: https://lore.kernel.org/linux-iommu/179018862538.3334538.17643821143626392419.b4-review@b4/ So, my PRI series would drop this patch. I suspect that it still has some value for the RPM/CMDQ case, compared to a busy-waiting alternative. Yet, it probably should be simplified for CMDQ case only. In RPM case, the "prod" would be fixed/stable right? IOW, there should be no new commands added to the cmdq, so it only needs to wait for cons == prod? Nicolin