Re: [RFC 0/3] i2c: sh_mobile: add DMA support

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Wolfram,

On Friday 31 October 2014 11:51:15 Wolfram Sang wrote:
> Here is my RFC to support DMA with the i2c-sh_mobile core. DMA works nicely
> with my tests so far and we save 1 interrupt per transferred byte, yay!

Do we have an idea of how much power (or CPU time ?) DMA support could save ?

> DMA is opt-in, so if setting it up fails, we will fall back to PIO. The
> threshold for selecting DMA is still under test, but probably good enough
> already. The major issue currently: This driver uses subsys_initcall() but
> at that time DMA is not available, and there is no deferred probe for DMA.
> So, switching to module_init() makes DMA work, but this may cause
> side-effects for older boards which rely on I2C being available early (to
> control some PMIC, for example). This needs some more investigation. Also,
> the driver (like all I2C DMA drivers currently) assumes that i2c message
> buffers are DMA capable. This is not always true and might need some
> assistance from the I2C core.

Given the amount of data we could probably use bounce buffers.

> Other than that, please test, review, comment. The series is based on
> renesas-devel-20141030-v3.18-rc2 with Laurent's series "[PATCH v4 0/5] R-Car
> Gen2 DMA Controller driver" on top of it. A git tree can be found here:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux.git
> renesas/i2c-shmobile-dma-experimental
> 
> Thanks,
> 
>     Wolfram
> 
> 
> Wolfram Sang (3):
>   i2c: sh_mobile: add DMA support
>   ARM: shmobile: r8a7790: add DMA nodes for IIC
>   ARM: shmobile: r8a7791: add DMA nodes for IIC
> 
>  .../devicetree/bindings/i2c/i2c-sh_mobile.txt      |   5 +
>  arch/arm/boot/dts/r8a7790.dtsi                     |   8 +
>  arch/arm/boot/dts/r8a7791.dtsi                     |   6 +
>  drivers/i2c/busses/i2c-sh_mobile.c                 | 203 ++++++++++++++++--
>  4 files changed, 203 insertions(+), 19 deletions(-)

-- 
Regards,

Laurent Pinchart