d4e01186ae
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com> Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
67 lines
3.4 KiB
Text
67 lines
3.4 KiB
Text
=================
|
|
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. It ensures data
|
|
is copied back to the message and a potentially used bounce buffer is freed::
|
|
|
|
i2c_release_dma_safe_msg_buf(msg, dma_buf);
|
|
|
|
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.
|