Re: [PATCH] i2c: rcar: add DMA support

[Wolfram Sang <wsa@the-dreams.de>]

Hi Niklas,

thanks for the submission. I was finally able to test this change.

On Wed, May 04, 2016 at 02:38:23PM +0200, Niklas Söderlund wrote:
> Make it possible to transfer i2c message buffers via DMA.
> Start/Stop/Sending_Slave_Address and some data is still handled using
> the old state machine, it is sending the bulk of the data that is done
> via DMA.
> 
> The first byte of a transmission and the last two bytes of reception are
> sent/received using PIO. This is needed for the HW to have access to the
> first byte before DMA transmit and to be able to set the STOP condition
> for DMA reception.
> 
> Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>

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

I did regression tests on my Salvator-X trying to trigger previously
known issues. Nothing bad happened. This could be expected since START
and STOP is done in PIO mode, but one never knows :) Also did verify
that DMA is triggered for bigger transfers.

Did you have time to re-measure the threshold? Also, did you try booting
without DMA and on Gen2? We had a bit of hazzle with !DMA with the
sh_mobile-driver. Boot test and basic i2cdetect will suffice.

Patch looks good, only minor nits:

> +	/* Do not use DMA for messages shorter then 8 bytes */
> +	if (msg->len < 8)
> +		return;
> +
> +	if (IS_ERR(chan))
> +		return;

Make this one if (a || b)?

> @@ -516,6 +738,7 @@ static int rcar_i2c_master_xfer(struct i2c_adapter *adap,
>  out:
>  	pm_runtime_put(dev);
>  
> +

Unrelated change ;)

Thanks,

   Wolfram