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Wed, 29 Jul 2026 09:47:46 -0700 Date: Wed, 29 Jul 2026 09:47:45 -0700 From: Nicolin Chen To: Pranjal Shrivastava CC: , , , , , , , , , , , , , , , , Subject: Re: [PATCH v2 03/11] iommu/arm-smmu-v3: Add arm_smmu_drain_queue_for_iopf() helper Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: X-NV-OnPremToCloud: ExternallySecured X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: MWH0EPF000C618B:EE_|DS2PR12MB9568:EE_ X-MS-Office365-Filtering-Correlation-Id: c29e170b-3b4a-4458-716a-08deed912a88 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|23010399003|82310400026|376014|7416014|36860700016|4143699003|56012099006|11063799006|10067099003|18002099003|22082099003; X-Microsoft-Antispam-Message-Info: 3j1m0/lUBb6nsH5GgN9EgYWE8wIldjnog6FWMtE95VuRL4xygL+c/Qs+2BDk8uizdAhqrGfvNzr95gZqH8LBAY5EFQdWr/H4SjBqh6La7n/K/2J9MQORXiL2ZFu3QI4ww4A1IFqDtsVcmh2zdfbZJ9BLWpMd7EqQ10SL8ptF/gASyYXqMQTygUQPZMYGWC0hAJuYJLvbP36Hq9QiAFVU4vai8xMSIkuVKA5IqkGRxPezyl9+GFocLpq3BIAKZzTJ8oL6pZtYZB+k6jQF/ZNxq8D0MCE2z0QBZliIkfCeSg0Kz55KqdvAINkWDeN+bZQVxd4IsQP6mMGiit5lVrZqrsOl5VkcdFILfggyugBJZZ7N/ikZrLE04YI2F3hgCNsOKZluAp09xOWO7rCjbxrBGqSjakVlruYdrEkVjKyR9GzD+bas7bHxyjfiJfLvl1P+EawdvnD4T7n4GvO5q2aGhALBVmR2UpUtnTYK65fIIL99PzTiEJSp4R2ha6LRnajQ7Mp4klXn5qc4jQnNZAKqKvzkNCnVsKusnHxm403g+mrz5fqgheK6N4MaVvHvr+eNi4xDhMrUemAWZnFzJIrxlKD8wObgvtaD/RWXA4SgDOUJWFJkQSVJwvVAhXeEbxSnOxGUpkfB9DwVM4WawFm983BTHXIZmuSblH6TTv+CQNPX+tUDXyfa30hCKZn20lvbLXUOml/Qt/upyB3s4M9FtA== X-Forefront-Antispam-Report: CIP:216.228.117.160;CTRY:US;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:mail.nvidia.com;PTR:dc6edge1.nvidia.com;CAT:NONE;SFS:(13230040)(1800799024)(23010399003)(82310400026)(376014)(7416014)(36860700016)(4143699003)(56012099006)(11063799006)(10067099003)(18002099003)(22082099003);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: vFiAvj98AqiA7ekeH8MhU3g5p+wLha41kWu1WcemhlDkkoCp5krNuilKgdVsec8DxiNUe72/xyse0DicGKKI7LpnK3sk8pNcyRnk1UXSYbbWSNXLYez6rSu+O4PVNAIZlti/v7k7lxh28DRXF1U6wwjKZoFwbpeJ1M7j5e5mSQu5bOyjnOHZVWcwg566npNhEnsyPbS0terGc+K6NX8b7BKpqsDqBcnmQ8IM11/zVJbubsRFMrPHqlue02QzkoQwqdKhgQLYkEdBza78UeQugJAVQ0VIQvje8CXRIsVI98n4S87MRTCkpk17Hhy4hDm4QDTsLxNBWX8JCS4JleOEONFyl0TniWwWkq79iVj1DxsSzy8nmtcFRleecIBTZPi4CqYBGzICUndtyoBvdjYxLDANRrtaCxvr+xATpGr/y2c8wBcPVHeGyO7uwRBgoc47 X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 29 Jul 2026 16:48:07.1436 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: c29e170b-3b4a-4458-716a-08deed912a88 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.117.160];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: MWH0EPF000C618B.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DS2PR12MB9568 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260729_094816_164376_6C08815F X-CRM114-Status: GOOD ( 16.30 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Jul 29, 2026 at 05:46:23AM +0000, Pranjal Shrivastava wrote: > On Tue, Jul 28, 2026 at 10:26:19PM -0700, Nicolin Chen wrote: > > On Wed, Jul 29, 2026 at 05:15:43AM +0000, Pranjal Shrivastava wrote: > > > For PRIQ/EVTQ, my only worry was the HW producing more events, > > > say Device A is detached (we call drain from attach_release) but Device B > > > (still attached) starts an agressive DMA with multiple PRI requests/evts. > > > > > > Maybe we should go back to snapshot based polling for that? > > > > If you look closely, "pending" is the snapshot, regardless of the > > growing prod. > > > > * With @until_empty == false (for EVTQ/PRIQ), exit once "drained" reaches its > > * target: "pending" (i.e. prod0 - cons0, frozen at the entry time): > > * > > * cons0 cons prod0 (prod) > > * |<---- drained ---->| | | > > * ---+###################+=====================+=============+---> > > * |<--------------- pending --------------->| > > > > Right, but we write back cons to cons_reg only after we see queue_empty() > We have a do { .. } while (!queue_empty(llq)); and then we call > queue_sync_cons_ovf(q). Same for PRIQ handler. > > IIUC the helper relies on cons_reg being updated: > > cons = readl_relaxed(q->cons_reg); > drained += Q_POS(&q->llq, cons - prev); > > Thus, if we get a storm of pri reqs / evts, we may never leave that loop > and queue_sync_cons_ovf(q). cons_reg is updated by queue_remove_raw() every iteration inside that loop. queue_sync_cons_ovf() doesn't update cons_reg unless an overflow. Nicolin