From patchwork Thu Aug 17 14:14:46 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Wolfram Sang X-Patchwork-Id: 9906407 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 B03896024A for ; Thu, 17 Aug 2017 14:15:48 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id A6DC128B05 for ; Thu, 17 Aug 2017 14:15:48 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 9BDEE28B10; Thu, 17 Aug 2017 14:15:48 +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 4B81B28B05 for ; Thu, 17 Aug 2017 14:15:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753212AbdHQOO6 (ORCPT ); Thu, 17 Aug 2017 10:14:58 -0400 Received: from sauhun.de ([88.99.104.3]:59941 "EHLO pokefinder.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753153AbdHQOOz (ORCPT ); Thu, 17 Aug 2017 10:14:55 -0400 Received: from localhost (p54B3365B.dip0.t-ipconnect.de [84.179.54.91]) by pokefinder.org (Postfix) with ESMTPSA id 4328E2C2F8F; Thu, 17 Aug 2017 16:14:53 +0200 (CEST) From: Wolfram Sang To: linux-i2c@vger.kernel.org Cc: linux-renesas-soc@vger.kernel.org, linux-kernel@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 v4 3/6] i2c: add docs to clarify DMA handling Date: Thu, 17 Aug 2017 16:14:46 +0200 Message-Id: <20170817141449.23958-4-wsa+renesas@sang-engineering.com> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20170817141449.23958-1-wsa+renesas@sang-engineering.com> References: <20170817141449.23958-1-wsa+renesas@sang-engineering.com> Sender: linux-input-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-input@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Signed-off-by: Wolfram Sang --- Documentation/i2c/DMA-considerations | 50 ++++++++++++++++++++++++++++++++++++ 1 file changed, 50 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..a4b4a107102452 --- /dev/null +++ b/Documentation/i2c/DMA-considerations @@ -0,0 +1,50 @@ +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. + +If you use such a buffer in a 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 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 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 either msg->buf for the I2C_M_DMA_SAFE case or a +bounce buffer. But you don't need to care about that detail. 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.