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. Another helper 54 function ensures a potentially used bounce buffer is freed:: 55 56 i2c_put_dma_safe_msg_buf(dma_buf, msg, xferred); 57 58 The last argument 'xferred' controls if the buffer is synced back to the 59 message or not. No syncing is needed in cases setting up DMA had an error and 60 there was no data transferred. 61 62 The bounce buffer handling from the core is generic and simple. It will always 63 allocate a new bounce buffer. If you want a more sophisticated handling (e.g. 64 reusing pre-allocated buffers), you are free to implement your own. 65 66 Please also check the in-kernel documentation for details. The i2c-sh_mobile 67 driver can be used as a reference example how to use the above helpers. 68 69 Final note: If you plan to use DMA with I2C (or with anything else, actually) 70 make sure you have CONFIG_DMA_API_DEBUG enabled during development. It can help 71 you find various issues which can be complex to debug otherwise.
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