Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752136AbdGRKYY (ORCPT ); Tue, 18 Jul 2017 06:24:24 -0400 Received: from sauhun.de ([88.99.104.3]:36517 "EHLO pokefinder.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751371AbdGRKYS (ORCPT ); Tue, 18 Jul 2017 06:24:18 -0400 From: Wolfram Sang To: linux-i2c@vger.kernel.org Cc: linux-renesas-soc@vger.kernel.org, linux-media@vger.kernel.org, linux-input@vger.kernel.org, linux-iio@vger.kernel.org, alsa-devel@alsa-project.org, linux-kernel@vger.kernel.org, Wolfram Sang Subject: [PATCH v3 2/4] i2c: add docs to clarify DMA handling Date: Tue, 18 Jul 2017 12:23:37 +0200 Message-Id: <20170718102339.28726-3-wsa+renesas@sang-engineering.com> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20170718102339.28726-1-wsa+renesas@sang-engineering.com> References: <20170718102339.28726-1-wsa+renesas@sang-engineering.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2434 Lines: 56 Signed-off-by: Wolfram Sang --- Changes since v2: * documentation updates. Hopefully better wording now Documentation/i2c/DMA-considerations | 38 ++++++++++++++++++++++++++++++++++++ 1 file changed, 38 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..e46c24d65c8556 --- /dev/null +++ b/Documentation/i2c/DMA-considerations @@ -0,0 +1,38 @@ +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. + +To support this scenario, drivers wishing to implement DMA can use helper +functions from the I2C core. One checks if a message is DMA capable in terms of +size and memory type. It can optionally also create a bounce buffer: + + i2c_check_msg_for_dma(msg, threshold, &bounce_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 can leave the pointer to the bounce buffer +empty and implement your own handling based on the return value of the above +function. + +The other helper function releases the bounce buffer. It ensures data is copied +back to the message: + + i2c_release_dma_bounce_buf(msg, bounce_buf); + +Please check the in-kernel documentation for details. The i2c-sh_mobile driver +can be used as a reference example. + +If you plan to use DMA with I2C (or with any other bus, 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. -- 2.11.0