=================
  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. Another helper
  function ensures a potentially used bounce buffer is freed::

  	i2c_put_dma_safe_msg_buf(dma_buf, msg, xferred);
  
  The last argument 'xferred' controls if the buffer is synced back to the
  message or not. No syncing is needed in cases setting up DMA had an error and
  there was no data transferred.

  
  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.