From patchwork Sat Nov 4 20:20:07 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Wolfram Sang X-Patchwork-Id: 834227 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Authentication-Results: ozlabs.org; spf=none (mailfrom) smtp.mailfrom=vger.kernel.org (client-ip=209.132.180.67; helo=vger.kernel.org; envelope-from=linux-i2c-owner@vger.kernel.org; receiver=) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by ozlabs.org (Postfix) with ESMTP id 3yTqyV4lcyz9sPt for ; Sun, 5 Nov 2017 07:23:14 +1100 (AEDT) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752880AbdKDUWh (ORCPT ); Sat, 4 Nov 2017 16:22:37 -0400 Received: from sauhun.de ([88.99.104.3]:44821 "EHLO pokefinder.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752172AbdKDUU1 (ORCPT ); Sat, 4 Nov 2017 16:20:27 -0400 Received: from localhost (p54B33782.dip0.t-ipconnect.de [84.179.55.130]) by pokefinder.org (Postfix) with ESMTPSA id B39902C35B5; Sat, 4 Nov 2017 21:20:24 +0100 (CET) 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, Mark Brown , Wolfram Sang , Jonathan Cameron , Mauro Carvalho Chehab Subject: [PATCH v6 7/9] i2c: add docs to clarify DMA handling Date: Sat, 4 Nov 2017 21:20:07 +0100 Message-Id: <20171104202009.3818-8-wsa+renesas@sang-engineering.com> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20171104202009.3818-1-wsa+renesas@sang-engineering.com> References: <20171104202009.3818-1-wsa+renesas@sang-engineering.com> Sender: linux-i2c-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-i2c@vger.kernel.org Reviewed-by: Jonathan Cameron Reviewed-by: Mauro Carvalho Chehab Signed-off-by: Wolfram Sang --- Documentation/i2c/DMA-considerations | 67 ++++++++++++++++++++++++++++++++++++ 1 file changed, 67 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..966610aa4620fa --- /dev/null +++ b/Documentation/i2c/DMA-considerations @@ -0,0 +1,67 @@ +================= +Linux I2C and DMA +================= + +Given that i2c is a low-speed bus, over which the majority of messages +transferred are small, 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. + +Clients +------- + +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. Also, when the core emulates +SMBus transactions via I2C, the buffers for block transfers are DMA safe. Users +of i2c_master_send() and i2c_master_recv() functions can now use DMA safe +variants (i2c_master_send_dmasafe() and i2c_master_recv_dmasafe()) once they +know their buffers are DMA safe. Users of i2c_transfer() must set the +I2C_M_DMA_SAFE flag manually. + +Masters +------- + +Bus master 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.