linux-kernel - Re: [PATCH v3 4/4] i2c: rcar: check for DMA-capable buffers

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Message-ID: <20170719094224.GO28538@bigcity.dyn.berto.se>
Date:   Wed, 19 Jul 2017 11:42:24 +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 4/4] i2c: rcar: check for DMA-capable buffers

Hi Wolfram,

On 2017-07-18 12:23:39 +0200, Wolfram Sang wrote:
> Handling this is special for this driver. Because the hardware needs to
> initialize the next message in interrupt context, we cannot use the
> i2c_check_msg_for_dma() directly. This helper only works reliably in
> process context. So, we need to check during initial preparation of the
> whole transfer and need to disable DMA completely for the whole transfer
> once a message with a not-DMA-capable buffer is found.
> 
> Signed-off-by: Wolfram Sang <wsa+renesas@...g-engineering.com>
> ---
>  drivers/i2c/busses/i2c-rcar.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-rcar.c b/drivers/i2c/busses/i2c-rcar.c
> index 93c1a54981df08..5d0e820d708853 100644
> --- a/drivers/i2c/busses/i2c-rcar.c
> +++ b/drivers/i2c/busses/i2c-rcar.c
> @@ -111,8 +111,11 @@
>  #define ID_ARBLOST	(1 << 3)
>  #define ID_NACK		(1 << 4)
>  /* persistent flags */
> +#define ID_P_NODMA	(1 << 30)
>  #define ID_P_PM_BLOCKED	(1 << 31)
> -#define ID_P_MASK	ID_P_PM_BLOCKED
> +#define ID_P_MASK	(ID_P_PM_BLOCKED | ID_P_NODMA)
> +
> +#define RCAR_DMA_THRESHOLD 8
>  
>  enum rcar_i2c_type {
>  	I2C_RCAR_GEN1,
> @@ -358,8 +361,7 @@ static void rcar_i2c_dma(struct rcar_i2c_priv *priv)
>  	unsigned char *buf;
>  	int len;
>  
> -	/* Do not use DMA if it's not available or for messages < 8 bytes */
> -	if (IS_ERR(chan) || msg->len < 8)
> +	if (IS_ERR(chan) || msg->len < RCAR_DMA_THRESHOLD || priv->flags & ID_P_NODMA)
>  		return;
>  
>  	if (read) {
> @@ -657,11 +659,15 @@ static void rcar_i2c_request_dma(struct rcar_i2c_priv *priv,
>  				 struct i2c_msg *msg)
>  {
>  	struct device *dev = rcar_i2c_priv_to_dev(priv);
> -	bool read;
> +	bool read = msg->flags & I2C_M_RD;
>  	struct dma_chan *chan;
>  	enum dma_transfer_direction dir;
>  
> -	read = msg->flags & I2C_M_RD;
> +	/* we need to check here because we need the 'current' context */

Maybe extend the comment here explaining that the check is primary here 
to make sure the msg->buf is valid for DMA and that the bounce buffer 
can't be used due to the special hardware feature? Else someone might be 
tempted to try and enable the bounce buffer feature in the future?

Nitpicking aside:

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

> +	if (i2c_check_msg_for_dma(msg, RCAR_DMA_THRESHOLD, NULL) == -EFAULT) {
> +		dev_dbg(dev, "skipping DMA for this whole transfer\n");
> +		priv->flags |= ID_P_NODMA;
> +	}
>  
>  	chan = read ? priv->dma_rx : priv->dma_tx;
>  	if (PTR_ERR(chan) != -EPROBE_DEFER)
> @@ -740,6 +746,8 @@ static int rcar_i2c_master_xfer(struct i2c_adapter *adap,
>  	if (ret < 0 && ret != -ENXIO)
>  		dev_err(dev, "error %d : %x\n", ret, priv->flags);
>  
> +	priv->flags &= ~ID_P_NODMA;
> +
>  	return ret;
>  }
>  
> -- 
> 2.11.0
> 

-- 
Regards,
Niklas Söderlund