From patchwork Thu Jun 15 18:30:37 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Wolfram Sang X-Patchwork-Id: 776392 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by ozlabs.org (Postfix) with ESMTP id 3wpXBx1wv8z9s7B for ; Fri, 16 Jun 2017 04:31:21 +1000 (AEST) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752872AbdFOSbG (ORCPT ); Thu, 15 Jun 2017 14:31:06 -0400 Received: from sauhun.de ([88.99.104.3]:38208 "EHLO pokefinder.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752891AbdFOSbA (ORCPT ); Thu, 15 Jun 2017 14:31:00 -0400 Received: from localhost (p54B338F0.dip0.t-ipconnect.de [84.179.56.240]) by pokefinder.org (Postfix) with ESMTPSA id E2AD32C319C; Thu, 15 Jun 2017 20:30:47 +0200 (CEST) From: Wolfram Sang To: linux-i2c@vger.kernel.org Cc: linux-renesas-soc@vger.kernel.org, linux-kernel@vger.kernel.org, Wolfram Sang , Wolfram Sang Subject: [PATCH 2/4] i2c: add docs to clarify DMA handling Date: Thu, 15 Jun 2017 20:30:37 +0200 Message-Id: <20170615183039.22925-3-wsa+renesas@sang-engineering.com> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20170615183039.22925-1-wsa+renesas@sang-engineering.com> References: <20170615183039.22925-1-wsa+renesas@sang-engineering.com> Sender: linux-i2c-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-i2c@vger.kernel.org Signed-off-by: Wolfram Sang Signed-off-by: Wolfram Sang --- Documentation/i2c/DMA-considerations | 37 ++++++++++++++++++++++++++++++++++++ 1 file changed, 37 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..92cdb6835ccf95 --- /dev/null +++ b/Documentation/i2c/DMA-considerations @@ -0,0 +1,37 @@ +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 other one releases the bounce buffer ensuring data is copied back to the +message: + + i2c_release_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. + +The bounce buffer handling from the core is generic and simple. It will always +allocate a new buffer. If you want a more sophisticated buffer handling (e.g. +reusing pre-allocated buffers), you can skip the generic handling from the core +and implement your own. + +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.