From patchwork Wed Sep 20 18:59:53 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Wolfram Sang X-Patchwork-Id: 9962081 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 4AA5C6056A for ; Wed, 20 Sep 2017 19:02:35 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 303EB289C5 for ; Wed, 20 Sep 2017 19:02:35 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 24055283AD; Wed, 20 Sep 2017 19:02:35 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=unavailable version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id AB914283AD for ; Wed, 20 Sep 2017 19:02:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751871AbdITTB0 (ORCPT ); Wed, 20 Sep 2017 15:01:26 -0400 Received: from sauhun.de ([88.99.104.3]:38838 "EHLO pokefinder.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751485AbdITTAF (ORCPT ); Wed, 20 Sep 2017 15:00:05 -0400 Received: from localhost (p54B332E6.dip0.t-ipconnect.de [84.179.50.230]) by pokefinder.org (Postfix) with ESMTPSA id EEA1F2C43A1; Wed, 20 Sep 2017 21:00:02 +0200 (CEST) From: Wolfram Sang To: linux-i2c@vger.kernel.org Cc: linux-kernel@vger.kernel.org, linux-renesas-soc@vger.kernel.org, linux-iio@vger.kernel.org, linux-input@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, Wolfram Sang Subject: [RFC PATCH v5 3/6] i2c: add docs to clarify DMA handling Date: Wed, 20 Sep 2017 20:59:53 +0200 Message-Id: <20170920185956.13874-4-wsa+renesas@sang-engineering.com> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20170920185956.13874-1-wsa+renesas@sang-engineering.com> References: <20170920185956.13874-1-wsa+renesas@sang-engineering.com> Sender: linux-media-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Signed-off-by: Wolfram Sang Reviewed-by: Mauro Carvalho Chehab Reviewed-by: Jonathan Cameron --- Documentation/i2c/DMA-considerations | 58 ++++++++++++++++++++++++++++++++++++ 1 file changed, 58 insertions(+) create mode 100644 Documentation/i2c/DMA-considerations diff --git a/Documentation/i2c/DMA-considerations b/Documentation/i2c/DMA-considerations new file mode 100644 index 00000000000000..5a63355c6a9b6f --- /dev/null +++ b/Documentation/i2c/DMA-considerations @@ -0,0 +1,58 @@ +================= +Linux I2C and DMA +================= + +Given that I2C is a low-speed bus where largely small messages are transferred, +it is not considered a prime user of DMA access. At this time of writing, only +10% of I2C bus master drivers have DMA support implemented. And the vast +majority of transactions are so small that setting up DMA for it will likely +add more overhead than a plain PIO transfer. + +Therefore, it is *not* mandatory that the buffer of an I2C message is DMA safe. +It does not seem reasonable to apply additional burdens when the feature is so +rarely used. However, it is recommended to use a DMA-safe buffer if your +message size is likely applicable for DMA. Most drivers have this threshold +around 8 bytes (as of today, this is mostly an educated guess, however). For +any message of 16 byte or larger, it is probably a really good idea. Please +note that other subsystems you use might add requirements. E.g., if your +I2C bus master driver is using USB as a bridge, then you need to have DMA +safe buffers always, because USB requires it. + +For clients, if you use a DMA safe buffer in i2c_msg, set the I2C_M_DMA_SAFE +flag with it. Then, the I2C core and drivers know they can safely operate DMA +on it. Note that using this flag is optional. I2C host drivers which are not +updated to use this flag will work like before. And like before, they risk +using an unsafe DMA buffer. To improve this situation, using I2C_M_DMA_SAFE in +more and more clients and host drivers is the planned way forward. Note also +that setting this flag makes only sense in kernel space. User space data is +copied into kernel space anyhow. The I2C core makes sure the destination +buffers in kernel space are always DMA capable. + +FIXME: Need to implement i2c_master_{send|receive}_dma and proper buffers for i2c_smbus_xfer_emulated. + +Drivers wishing to implement safe DMA can use helper functions from the I2C +core. One gives you a DMA-safe buffer for a given i2c_msg as long as a certain +threshold is met:: + + dma_buf = i2c_get_dma_safe_msg_buf(msg, threshold_in_byte); + +If a buffer is returned, it is either msg->buf for the I2C_M_DMA_SAFE case or a +bounce buffer. But you don't need to care about that detail, just use the +returned buffer. If NULL is returned, the threshold was not met or a bounce +buffer could not be allocated. Fall back to PIO in that case. + +In any case, a buffer obtained from above needs to be released. It ensures data +is copied back to the message and a potentially used bounce buffer is freed:: + + i2c_release_dma_safe_msg_buf(msg, dma_buf); + +The bounce buffer handling from the core is generic and simple. It will always +allocate a new bounce buffer. If you want a more sophisticated handling (e.g. +reusing pre-allocated buffers), you are free to implement your own. + +Please also check the in-kernel documentation for details. The i2c-sh_mobile +driver can be used as a reference example how to use the above helpers. + +Final note: If you plan to use DMA with I2C (or with anything else, actually) +make sure you have CONFIG_DMA_API_DEBUG enabled during development. It can help +you find various issues which can be complex to debug otherwise.