Signed-off-by: Wolfram Sang <wsa+renesas@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Wolfram Sang <wsa@xxxxxxxxxxxxx> --- 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. -- 2.11.0 -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
