Signed-off-by: Wolfram Sang <wsa+renesas@xxxxxxxxxxxxxxxxxxxx> --- Documentation/i2c/DMA-considerations | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 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..945ac5cfe55c12 --- /dev/null +++ b/Documentation/i2c/DMA-considerations @@ -0,0 +1,28 @@ +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 (FIXME: > 8 byte?). + +To support this, drivers wishing to implement DMA can use a helper function +checking if the size is suitable for DMA or if the buffer is DMA capable: + + int i2c_check_msg_for_dma(msg, dma_threshold); + +Please check its in kernel documentation for details. + +It should be further noted that bounce buffer handling is left to be handled on +driver level because details like alignment requirements are best known on that +level. + +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
