On Tue, 18 Jul 2017 12:23:37 +0200 Wolfram Sang <wsa+renesas@xxxxxxxxxxxxxxxxxxxx> wrote: > Signed-off-by: Wolfram Sang <wsa+renesas@xxxxxxxxxxxxxxxxxxxx> Is this material not perhaps better placed in the sphinx docs? Up to you of course as your subsystem ;) Text is good though. Acked-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> > --- > 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.
