linux-kernel - Re: [PATCH v3 3/4] i2c: sh_mobile: use helper to decide if DMA is useful

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Date:   Wed, 19 Jul 2017 11:35:45 +0200
From:   Niklas Söderlund 
        <niklas.soderlund@...natech.se>
To:     Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc:     linux-i2c@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        linux-media@...r.kernel.org, linux-input@...r.kernel.org,
        linux-iio@...r.kernel.org, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/4] i2c: sh_mobile: use helper to decide if DMA is
 useful

Hi Wolfram,

On 2017-07-18 12:23:38 +0200, Wolfram Sang wrote:
> This ensures that we fall back to PIO if the buffer is too small for DMA
> being useful. Otherwise, we use DMA. A bounce buffer might be applied if
> the original message buffer is not DMA safe
> 
> Signed-off-by: Wolfram Sang <wsa+renesas@...g-engineering.com>
> ---
>  drivers/i2c/busses/i2c-sh_mobile.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-sh_mobile.c b/drivers/i2c/busses/i2c-sh_mobile.c
> index 2e097d97d258bc..19f45bcd9b35ca 100644
> --- a/drivers/i2c/busses/i2c-sh_mobile.c
> +++ b/drivers/i2c/busses/i2c-sh_mobile.c
> @@ -145,6 +145,7 @@ struct sh_mobile_i2c_data {
>  	struct dma_chan *dma_rx;
>  	struct scatterlist sg;
>  	enum dma_data_direction dma_direction;
> +	u8 *bounce_buf;
>  };
>  
>  struct sh_mobile_dt_config {
> @@ -548,6 +549,8 @@ static void sh_mobile_i2c_dma_callback(void *data)
>  	pd->pos = pd->msg->len;
>  	pd->stop_after_dma = true;
>  
> +	i2c_release_dma_bounce_buf(pd->msg, pd->bounce_buf);
> +
>  	iic_set_clr(pd, ICIC, 0, ICIC_TDMAE | ICIC_RDMAE);
>  }
>  
> @@ -595,6 +598,7 @@ static void sh_mobile_i2c_xfer_dma(struct sh_mobile_i2c_data *pd)
>  	struct dma_async_tx_descriptor *txdesc;
>  	dma_addr_t dma_addr;
>  	dma_cookie_t cookie;
> +	u8 *dma_buf = pd->bounce_buf ?: pd->msg->buf;

This looked funny and I had to look it up, I learnt something new today 
:-)

>  
>  	if (PTR_ERR(chan) == -EPROBE_DEFER) {
>  		if (read)
> @@ -608,7 +612,7 @@ static void sh_mobile_i2c_xfer_dma(struct sh_mobile_i2c_data *pd)
>  	if (IS_ERR(chan))
>  		return;
>  
> -	dma_addr = dma_map_single(chan->device->dev, pd->msg->buf, pd->msg->len, dir);
> +	dma_addr = dma_map_single(chan->device->dev, dma_buf, pd->msg->len, dir);
>  	if (dma_mapping_error(chan->device->dev, dma_addr)) {
>  		dev_dbg(pd->dev, "dma map failed, using PIO\n");
>  		return;
> @@ -665,7 +669,7 @@ static int start_ch(struct sh_mobile_i2c_data *pd, struct i2c_msg *usr_msg,
>  	pd->pos = -1;
>  	pd->sr = 0;
>  
> -	if (pd->msg->len > 8)
> +	if (i2c_check_msg_for_dma(pd->msg, 8, &pd->bounce_buf) == 0)

Maybe the 8 should be declared in a define to explain the value, like 
you do in patch 4/4?

This nitpick aside:

Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>

>  		sh_mobile_i2c_xfer_dma(pd);
>  
>  	/* Enable all interrupts to begin with */
> -- 
> 2.11.0
> 

-- 
Regards,
Niklas Söderlund