|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.
|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.
|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.