72 lines
3.6 KiB
ReStructuredText
72 lines
3.6 KiB
ReStructuredText
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=================
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Linux I2C and DMA
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=================
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Given that I2C is a low-speed bus, over which the majority of messages
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transferred are small, it is not considered a prime user of DMA access. At this
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time of writing, only 10% of I2C bus master drivers have DMA support
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implemented. And the vast majority of transactions are so small that setting up
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DMA for it will likely add more overhead than a plain PIO transfer.
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Therefore, it is *not* mandatory that the buffer of an I2C message is DMA safe.
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It does not seem reasonable to apply additional burdens when the feature is so
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rarely used. However, it is recommended to use a DMA-safe buffer if your
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message size is likely applicable for DMA. Most drivers have this threshold
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around 8 bytes (as of today, this is mostly an educated guess, however). For
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any message of 16 byte or larger, it is probably a really good idea. Please
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note that other subsystems you use might add requirements. E.g., if your
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I2C bus master driver is using USB as a bridge, then you need to have DMA
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safe buffers always, because USB requires it.
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Clients
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-------
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For clients, if you use a DMA safe buffer in i2c_msg, set the I2C_M_DMA_SAFE
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flag with it. Then, the I2C core and drivers know they can safely operate DMA
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on it. Note that using this flag is optional. I2C host drivers which are not
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updated to use this flag will work like before. And like before, they risk
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using an unsafe DMA buffer. To improve this situation, using I2C_M_DMA_SAFE in
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more and more clients and host drivers is the planned way forward. Note also
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that setting this flag makes only sense in kernel space. User space data is
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copied into kernel space anyhow. The I2C core makes sure the destination
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buffers in kernel space are always DMA capable. Also, when the core emulates
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SMBus transactions via I2C, the buffers for block transfers are DMA safe. Users
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of i2c_master_send() and i2c_master_recv() functions can now use DMA safe
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variants (i2c_master_send_dmasafe() and i2c_master_recv_dmasafe()) once they
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know their buffers are DMA safe. Users of i2c_transfer() must set the
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I2C_M_DMA_SAFE flag manually.
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Masters
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-------
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Bus master drivers wishing to implement safe DMA can use helper functions from
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the I2C core. One gives you a DMA-safe buffer for a given i2c_msg as long as a
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certain threshold is met::
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dma_buf = i2c_get_dma_safe_msg_buf(msg, threshold_in_byte);
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If a buffer is returned, it is either msg->buf for the I2C_M_DMA_SAFE case or a
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bounce buffer. But you don't need to care about that detail, just use the
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returned buffer. If NULL is returned, the threshold was not met or a bounce
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buffer could not be allocated. Fall back to PIO in that case.
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In any case, a buffer obtained from above needs to be released. Another helper
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function ensures a potentially used bounce buffer is freed::
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i2c_put_dma_safe_msg_buf(dma_buf, msg, xferred);
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The last argument 'xferred' controls if the buffer is synced back to the
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message or not. No syncing is needed in cases setting up DMA had an error and
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there was no data transferred.
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The bounce buffer handling from the core is generic and simple. It will always
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allocate a new bounce buffer. If you want a more sophisticated handling (e.g.
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reusing pre-allocated buffers), you are free to implement your own.
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Please also check the in-kernel documentation for details. The i2c-sh_mobile
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driver can be used as a reference example how to use the above helpers.
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Final note: If you plan to use DMA with I2C (or with anything else, actually)
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make sure you have CONFIG_DMA_API_DEBUG enabled during development. It can help
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you find various issues which can be complex to debug otherwise.
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