[RFC PATCH v4 0/6] i2c: document DMA handling and add helpers for it

[Jonathan Cameron]

On Thu, 17 Aug 2017 16:14:43 +0200
Wolfram Sang <wsa+renesas at sang-engineering.com> wrote:

> So, after revisiting old mail threads, taking part in a similar discussion on
> the USB list, and implementing a not-convincing solution before, here is what I
> cooked up to document and ease DMA handling for I2C within Linux. Please have a
> look at the documentation introduced in patch 3 for details.
> 
> While the previous versions tried to magically apply bounce buffers when
> needed, it became clear that detecting DMA safe buffers is too fragile. This
> approach is now opt-in, a DMA_SAFE flag needs to be set on an i2c_msg. The
> outcome so far is very convincing IMO. The core additions are simple and easy
> to understand (makes me even think of inlining them again?). The driver changes
> for the Renesas IP cores became easier to understand, too. While only a tad for
> the i2c-sh_mobile driver, the situation became a LOT better for the i2c-rcar
> driver. No more DMA disabling for the whole transfer in case of unsafe buffers,
> we are back to per-msg handling. And the code fix is now an easy to understand
> one line change. Yay!
> 
> Of course, we must now whitelist DMA safe buffers. An example for I2C_RDWR case
> is in this series. It makes the i2ctransfer utility have DMA_SAFE buffers,
> which is nice for testing as i2cdump will (currently) not use DMA_SAFE buffers.
> My plan is to add two new calls: i2c_master_{send|receive}_dma_safe which can
> be used if DMA_SAFE buffers are provided. So, drivers can simply switch to
> them. Also, the buffers used within i2c_smbus_xfer_emulated() need to be
> converted to be DMA_SAFE which will cover a huge bunch of use cases. The rest
> is then updating drivers which can be done when needed.
> 
> As these conversions are not done yet, this patch series has RFC status. But I
> already would like to get opinions on this approach, so I'll cc mailing lists
> of the heavier I2C users. Please let me know what you think.
> 
> All patches have been tested with a Renesas Salvator-X board (r8a7796/M3-W).
> 
> The branch can be found here:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux.git renesas/topic/i2c-core-dma-rfc-v4
> 
> And big kudos to Renesas Electronics for funding this work, thank you very much!

All looks good to me.  I should really set up some testing for this to see if it
gains us much on the various platforms I use, but that will 'a while' so don't wait
on me!  This is particularly true as none of them have dma support in the master
drivers yet.

Thanks for you work on this and cool than Renesas are funding it!

Jonathan

> 
> Regards,
> 
>    Wolfram
> 
> Changes since v3:
> 	* completely redesigned
> 
> Wolfram Sang (6):
>   i2c: add a message flag for DMA safe buffers
>   i2c: add helpers to ease DMA handling
>   i2c: add docs to clarify DMA handling
>   i2c: sh_mobile: use helper to decide if DMA is useful
>   i2c: rcar: skip DMA if buffer is not safe
>   i2c: dev: mark RDWR buffers as DMA_SAFE
> 
>  Documentation/i2c/DMA-considerations | 50 ++++++++++++++++++++++++++++++++++++
>  drivers/i2c/busses/i2c-rcar.c        |  2 +-
>  drivers/i2c/busses/i2c-sh_mobile.c   |  8 ++++--
>  drivers/i2c/i2c-core-base.c          | 45 ++++++++++++++++++++++++++++++++
>  drivers/i2c/i2c-dev.c                |  2 ++
>  include/linux/i2c.h                  |  3 +++
>  include/uapi/linux/i2c.h             |  3 +++
>  7 files changed, 110 insertions(+), 3 deletions(-)
>  create mode 100644 Documentation/i2c/DMA-considerations
>