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| 1 | +================= |
| 2 | +Linux I2C and DMA |
| 3 | +================= |
| 4 | + |
| 5 | +Given that i2c is a low-speed bus, over which the majority of messages |
| 6 | +transferred are small, it is not considered a prime user of DMA access. At this |
| 7 | +time of writing, only 10% of I2C bus master drivers have DMA support |
| 8 | +implemented. And the vast majority of transactions are so small that setting up |
| 9 | +DMA for it will likely add more overhead than a plain PIO transfer. |
| 10 | + |
| 11 | +Therefore, it is *not* mandatory that the buffer of an I2C message is DMA safe. |
| 12 | +It does not seem reasonable to apply additional burdens when the feature is so |
| 13 | +rarely used. However, it is recommended to use a DMA-safe buffer if your |
| 14 | +message size is likely applicable for DMA. Most drivers have this threshold |
| 15 | +around 8 bytes (as of today, this is mostly an educated guess, however). For |
| 16 | +any message of 16 byte or larger, it is probably a really good idea. Please |
| 17 | +note that other subsystems you use might add requirements. E.g., if your |
| 18 | +I2C bus master driver is using USB as a bridge, then you need to have DMA |
| 19 | +safe buffers always, because USB requires it. |
| 20 | + |
| 21 | +Clients |
| 22 | +------- |
| 23 | + |
| 24 | +For clients, if you use a DMA safe buffer in i2c_msg, set the I2C_M_DMA_SAFE |
| 25 | +flag with it. Then, the I2C core and drivers know they can safely operate DMA |
| 26 | +on it. Note that using this flag is optional. I2C host drivers which are not |
| 27 | +updated to use this flag will work like before. And like before, they risk |
| 28 | +using an unsafe DMA buffer. To improve this situation, using I2C_M_DMA_SAFE in |
| 29 | +more and more clients and host drivers is the planned way forward. Note also |
| 30 | +that setting this flag makes only sense in kernel space. User space data is |
| 31 | +copied into kernel space anyhow. The I2C core makes sure the destination |
| 32 | +buffers in kernel space are always DMA capable. Also, when the core emulates |
| 33 | +SMBus transactions via I2C, the buffers for block transfers are DMA safe. Users |
| 34 | +of i2c_master_send() and i2c_master_recv() functions can now use DMA safe |
| 35 | +variants (i2c_master_send_dmasafe() and i2c_master_recv_dmasafe()) once they |
| 36 | +know their buffers are DMA safe. Users of i2c_transfer() must set the |
| 37 | +I2C_M_DMA_SAFE flag manually. |
| 38 | + |
| 39 | +Masters |
| 40 | +------- |
| 41 | + |
| 42 | +Bus master drivers wishing to implement safe DMA can use helper functions from |
| 43 | +the I2C core. One gives you a DMA-safe buffer for a given i2c_msg as long as a |
| 44 | +certain threshold is met:: |
| 45 | + |
| 46 | + dma_buf = i2c_get_dma_safe_msg_buf(msg, threshold_in_byte); |
| 47 | + |
| 48 | +If a buffer is returned, it is either msg->buf for the I2C_M_DMA_SAFE case or a |
| 49 | +bounce buffer. But you don't need to care about that detail, just use the |
| 50 | +returned buffer. If NULL is returned, the threshold was not met or a bounce |
| 51 | +buffer could not be allocated. Fall back to PIO in that case. |
| 52 | + |
| 53 | +In any case, a buffer obtained from above needs to be released. It ensures data |
| 54 | +is copied back to the message and a potentially used bounce buffer is freed:: |
| 55 | + |
| 56 | + i2c_release_dma_safe_msg_buf(msg, dma_buf); |
| 57 | + |
| 58 | +The bounce buffer handling from the core is generic and simple. It will always |
| 59 | +allocate a new bounce buffer. If you want a more sophisticated handling (e.g. |
| 60 | +reusing pre-allocated buffers), you are free to implement your own. |
| 61 | + |
| 62 | +Please also check the in-kernel documentation for details. The i2c-sh_mobile |
| 63 | +driver can be used as a reference example how to use the above helpers. |
| 64 | + |
| 65 | +Final note: If you plan to use DMA with I2C (or with anything else, actually) |
| 66 | +make sure you have CONFIG_DMA_API_DEBUG enabled during development. It can help |
| 67 | +you find various issues which can be complex to debug otherwise. |
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