Re: [RFC PATCH v5 3/6] i2c: add docs to clarify DMA handling

[Mauro Carvalho Chehab <mchehab@s-opensource.com>]

Em Wed, 20 Sep 2017 20:59:53 +0200
Wolfram Sang <wsa+renesas@sang-engineering.com> escreveu:

> Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>

Documentation looks OK on my eyes. So:

Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

> ---
>  Documentation/i2c/DMA-considerations | 58 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 58 insertions(+)
>  create mode 100644 Documentation/i2c/DMA-considerations
> 
> diff --git a/Documentation/i2c/DMA-considerations b/Documentation/i2c/DMA-considerations
> new file mode 100644
> index 00000000000000..5a63355c6a9b6f
> --- /dev/null
> +++ b/Documentation/i2c/DMA-considerations
> @@ -0,0 +1,58 @@
> +=================
> +Linux I2C and DMA
> +=================
> +
> +Given that I2C is a low-speed bus where largely small messages are transferred,
> +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.
> +
> +FIXME: Need to implement i2c_master_{send|receive}_dma and proper buffers for i2c_smbus_xfer_emulated.
> +
> +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.



Thanks,
Mauro