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

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Message-ID: <CAKocOONjafedew70jMnV9HfNru6mhQV6FXAkoHTL3wzght7+Ww@mail.gmail.com>
Date:   Thu, 15 Jun 2017 12:48:22 -0600
From:   Shuah Khan <shuahkhan@...il.com>
To:     Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc:     linux-i2c@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Wolfram Sang <wsa@...-dreams.de>, shuah@...nel.org
Subject: Re: [PATCH 4/4] i2c: rcar: check for DMA-capable buffers

On Thu, Jun 15, 2017 at 12:30 PM, Wolfram Sang
<wsa+renesas@...g-engineering.com> 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>
> Signed-off-by: Wolfram Sang <wsa@...-dreams.de>
> ---
>  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 8be3e6cb8fe686..2b679ba4b75066 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)

Might be more efficient to check for ID_P_NODMA first instead of msg->len.

>                 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 */
> +       if (i2c_check_msg_for_dma(msg, RCAR_DMA_THRESHOLD, NULL) == -EFAULT) {
> +               dev_dbg(dev, "skipping DMA for this whole transfer\n");

Adding reason for skipping will be helpful.

> +               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
>

thanks,
-- Shuah