So, after revisiting old mail threads and taking part in a similar discussion
on the USB list, 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 2
for further details.

All patches have been tested with a Renesas Salvator-X board (r8a7796/M3-W) and
a Renesas Lager board (r8a7790/H2). A more detailed test description can be
found here: http://elinux.org/Tests:I2C-core-DMA

The branch can be found here:

git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux.git renesas/topic/i2c-core-dma-v3

And big kudos to Renesas Electronics for funding this work, thank you very much!

Regards,

   Wolfram


Changes since v2:

* rebased to v4.13-rc1
* helper functions are not inlined anymore but moved to i2c core
* __must_check has been added to the buffer check helper
* the release function has been renamed to contain 'dma' as well
* documentation updates. Hopefully better wording now
* removed the doubled Signed-offs
* adding more potentially interested parties to CC


Wolfram Sang (4):
  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: check for DMA-capable buffers

 Documentation/i2c/DMA-considerations | 38 ++++++++++++++++++++
 drivers/i2c/busses/i2c-rcar.c        | 18 +++++++---
 drivers/i2c/busses/i2c-sh_mobile.c   |  8 +++--
 drivers/i2c/i2c-core-base.c          | 68 ++++++++++++++++++++++++++++++++++++
 include/linux/i2c.h                  |  5 +++
 5 files changed, 130 insertions(+), 7 deletions(-)
 create mode 100644 Documentation/i2c/DMA-considerations

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2.11.0