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b=VkO5qbtTB02E6wi53ItyhdJjouNz83UC33k8aXUyr5fBIOuoEL55zJZDi/6NHCWEl580tXqlBO14ZKvX/uVWWFDo6KNv7Z7WNCmI8nUKlcENJEqnKXK/DSHpzuy9hq5UqTudRCyrUleVB0uEV+rdFRa7o51UpjmTr1mUUUcMq1E= Received: from PAXPR04MB9642.eurprd04.prod.outlook.com (2603:10a6:102:240::14) by AM9PR04MB8398.eurprd04.prod.outlook.com (2603:10a6:20b:3b7::19) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.7409.46; Mon, 1 Apr 2024 17:42:17 +0000 Received: from PAXPR04MB9642.eurprd04.prod.outlook.com ([fe80::3168:91:27c6:edf6]) by PAXPR04MB9642.eurprd04.prod.outlook.com ([fe80::3168:91:27c6:edf6%3]) with mapi id 15.20.7409.042; Mon, 1 Apr 2024 17:42:17 +0000 From: Frank Li To: rdunlap@infradead.org, hch@infradead.org Cc: Frank.Li@nxp.com, corbet@lwn.net, dmaengine@vger.kernel.org, imx@lists.linux.dev, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org, lizhijian@fujitsu.com, mst@redhat.com Subject: [PATCH v2 1/1] docs: dma: correct dma_set_mask() sample code Date: Mon, 1 Apr 2024 13:41:59 -0400 Message-Id: <20240401174159.642998-1-Frank.Li@nxp.com> X-Mailer: git-send-email 2.34.1 X-ClientProxiedBy: BY5PR16CA0001.namprd16.prod.outlook.com (2603:10b6:a03:1a0::14) To PAXPR04MB9642.eurprd04.prod.outlook.com (2603:10a6:102:240::14) Precedence: bulk X-Mailing-List: linux-pci@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PAXPR04MB9642:EE_|AM9PR04MB8398:EE_ X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: Xafy3gIFJS4y/PrNThQh9F0iBsMNhw894eqTmp+kd4QagJWlFjFrXBpzmk2AKU04L/lLJhN2ev1ZkREZvZprAdofA9Zted6fnpRz8x2S6uu3UZx5WCh6bQL4NRU1c/n8dOx5iYAAfwCF15DqeBnasV8bYLCxgSPT3UplswwncheJucgFhdt2ECKLyCiLxluaCVbV7fjEiJY0DYhwA5GkoxKiGDKqZInjNKuFK7Awq4RXENwSY5xHgsPU9Pd6EWIzklrmLGd8Dan8w4umC0pf19LAHc0JFFWseFvC9fAcNiS3rE9Nt6FQSNt0jRFw0buuI31YUTajXxxmOPy9UzuxIHJoWwJXsQq9PYIvu7yYhpzU7a6/Qg2TjuPQEFZRWyGau99SZCuq6MdfmlS9vAUgMoDjmKUdReFcgOf66+S/WvkOC7LEW0nRSQK1+QB8OtFXcFIKSwFBRWA1iYHbN6/RJSAJ46ZvMRdXb+eV/gnUum0f0f4QBGvtyaKRC1GOdH7NjfXFkrp3k/179NXJQ0DzLYB1yScsZtrIsE0r/U+pid1yiijeQE0BpnNg46l75b+n84z3eA/k71fuOnrrI3PQZ3KNUatSlDKwi+X5FDRmVYNRsJNQx1FVmYuWkehtkdZLuk6wJqqDpFvz6nNVdxmn1wyVhuMKF2XBHKXlxYSITBx2w+awZvZ4ZKvDzVf5uP1KBc9itMCASXJq2sO9XpMEnFJ8O7NfMjn2sW9NQhJVXOBrzCEVWgg6DxYJ9ACKvrrKMmWbmpJWIQzDsM9BRb6HuH/Lr/zrJF+uNUNWFZh1+mc6YNafHdTMwNwIUjxhK//KVc18hHVhA8tTQDMKIsvGtYaBPwKHz3G9fb63Fibjq+6naruZf+RQ3XkLZ2dvuSAh8ODo31c8BDBp6JJtb9FXmYIdTfCyDf7+SkwYHoo6Ynk5o5V1V+qYKqhzvvhDvPtfT3g9QF0YoA9WkiN0qBsxvCSId9dYpkn0wbMzXaiSNPl1quMIbc2w1KQIaSCM377ZfMHASHoUs7takrNdhRmUnud/A4ciWdVQSw0bqS9kPYGhF8lbeQnbRsqAvBGvK/v9x2epBJASBvoLcIJEqIvBFyn9idKDrAvadQb/LGUZMa5Im+OdNlMo0FCDPW9qIWl+6/4VDhaPqAuVFfuubV4uD8F8IllzRJrGVURm0RqYhEd8LwXmlC/1nkfzEj+AIqcMAiNZdrWzzgKncN9GBvUHp4v2NXBsOw3t9yHOf9j0vCDQMEGIs/6/QyhHQm6IjAETx0ZFkDOBECw1vmCbBgpDQcMsN7XEzxkx0E0ZGL2mChaE+mYGZfrgWpIH776H+LWsrQm3fJOdaJiCxvRFVl37wX5+PmqZ7GJj/aB+cPzxuxS/eyMgDB01jVN/zeiMZYfieGWzliqXYLb8/UXemqNUUUK86b2JYS+TZsgyHU1NpPc9pfdP35Ti95NTEi087cZS7+cRJ/LbK3gTLb9usjdkkOadcR4yf9AT24kHffA/zaclhXfIAVzRT6FRZQh0oL9aejYxO6p1VIk7UBFqDNy5M0uQe91JqaIw4b1LG9DNwr5FrB/P87hvQGG8A7SgD2Pv X-OriginatorOrg: nxp.com X-MS-Exchange-CrossTenant-Network-Message-Id: 986ed8c2-89a9-4861-caa4-08dc527312cc X-MS-Exchange-CrossTenant-AuthSource: PAXPR04MB9642.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 01 Apr 2024 17:42:17.1251 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 686ea1d3-bc2b-4c6f-a92c-d99c5c301635 X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: XUfCZDaSu12yqLpc6imfsFY9s/sa1cRDplUn/30/ykxmsD8gncXgD5Mw9WzfPY3gtezel+sH7OeQtLiJvcK89Q== X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM9PR04MB8398 There are bunch of codes in driver like if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))) dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32)) Actually it is wrong because if dma_set_mask_and_coherent(64) fails, dma_set_mask_and_coherent(32) will fail for the same reason. And dma_set_mask_and_coherent(64) never returns failure. According to the definition of dma_set_mask(), it indicates the width of address that device DMA can access. If it can access 64-bit address, it must access 32-bit address inherently. So only need set biggest address width. See below code fragment: dma_set_mask(mask) { mask = (dma_addr_t)mask; if (!dev->dma_mask || !dma_supported(dev, mask)) return -EIO; arch_dma_set_mask(dev, mask); *dev->dma_mask = mask; return 0; } dma_supported() will call dma_direct_supported or iommux's dma_supported call back function. int dma_direct_supported(struct device *dev, u64 mask) { u64 min_mask = (max_pfn - 1) << PAGE_SHIFT; /* * Because 32-bit DMA masks are so common we expect every architecture * to be able to satisfy them - either by not supporting more physical * memory, or by providing a ZONE_DMA32. If neither is the case, the * architecture needs to use an IOMMU instead of the direct mapping. */ if (mask >= DMA_BIT_MASK(32)) return 1; ... } The iommux's dma_supported() actually means iommu requires devices's minimized dma capability. An example: static int sba_dma_supported( struct device *dev, u64 mask)() { ... * check if mask is >= than the current max IO Virt Address * The max IO Virt address will *always* < 30 bits. */ return((int)(mask >= (ioc->ibase - 1 + (ioc->pdir_size / sizeof(u64) * IOVP_SIZE) ))); ... } 1 means supported. 0 means unsupported. Correct document to make it more clear and provide correct sample code. Signed-off-by: Frank Li Reviewed-by: Christoph Hellwig --- Notes: Change from v1 to v2: - fixed typo, review by Randy Dunlap Documentation/core-api/dma-api-howto.rst | 24 ++++++++++++++++++++++-- 1 file changed, 22 insertions(+), 2 deletions(-) diff --git a/Documentation/core-api/dma-api-howto.rst b/Documentation/core-api/dma-api-howto.rst index e8a55f9d61dbc..5f6a7d86b6bc2 100644 --- a/Documentation/core-api/dma-api-howto.rst +++ b/Documentation/core-api/dma-api-howto.rst @@ -203,13 +203,33 @@ setting the DMA mask fails. In this manner, if a user of your driver reports that performance is bad or that the device is not even detected, you can ask them for the kernel messages to find out exactly why. -The standard 64-bit addressing device would do something like this:: +The 24-bit addressing device would do something like this:: - if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))) { + if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(24))) { dev_warn(dev, "mydev: No suitable DMA available\n"); goto ignore_this_device; } +The standard 64-bit addressing device would do something like this:: + + dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64)) + +dma_set_mask_and_coherent() never return fail when DMA_BIT_MASK(64). Typical +error code like:: + + /* Wrong code */ + if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))) + dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32)) + +dma_set_mask_and_coherent() will never return failure when bigger then 32. +So typical code like:: + + /* Recommended code */ + if (support_64bit) + dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64)); + else + dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32)); + If the device only supports 32-bit addressing for descriptors in the coherent allocations, but supports full 64-bits for streaming mappings it would look like this::