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

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Wed, 20 Sep 2017 16:56:48 -0300
Mauro Carvalho Chehab <mchehab@s-opensource.com> wrote:

> 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>

Really minor suggestion inline. I don't really care either way as
what you had is perfectly comprehensible. 

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.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,

Slightly nicer as:

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.
> > +
> > +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
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