[PATCH] i2c-s3c2410: Remove unconditional 1ms delay on each transfer

[jassi brar]

On Fri, Apr 2, 2010 at 9:39 PM, Mark Brown
<broonie at opensource.wolfsonmicro.com> wrote:
> The S3C I2C controller indicates completion of I2C transfers before
> the bus has a stop condition on it. In order to ensure that we do not
> attempt to start a new transfer before the bus is idle the driver
> currently inserts a 1ms delay. This is vastly larger than is generally
> required and has a visible effect on performance under load, such as
> when bringing up audio CODECs or reading back status information with
> non-bulk I2C reads.
>
> Replace the sleep with a spin on the IIC status register for up to 1ms.
> This will busy wait but testing on my SMDK6410 system indicates that
> the overwhelming majority of transactions complete on the first spin,
> with maximum latencies of less than 10 spins so the absolute overhead
> of busy waiting should be at worst comprable to msleep(), and the
> overall system performance is dramatically improved.
>
> The main risk is poor interaction with multimaster systems where
> we may miss the bus going idle before the next transaction. Defend
> against this by falling back to the original 1ms delay after 20 spins.
>
> The overall effect in my testing is an approximately 20% improvement
> in kernel startup time.
>
> Signed-off-by: Mark Brown <broonie at opensource.wolfsonmicro.com>
> ---
>  drivers/i2c/busses/i2c-s3c2410.c |   20 ++++++++++++++++++--
>  1 files changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/i2c/busses/i2c-s3c2410.c b/drivers/i2c/busses/i2c-s3c2410.c
> index d27072b..ba52543 100644
> --- a/drivers/i2c/busses/i2c-s3c2410.c
> +++ b/drivers/i2c/busses/i2c-s3c2410.c
> @@ -482,7 +482,8 @@ static int s3c24xx_i2c_set_master(struct s3c24xx_i2c *i2c)
>  static int s3c24xx_i2c_doxfer(struct s3c24xx_i2c *i2c,
>                              struct i2c_msg *msgs, int num)
>  {
> -       unsigned long timeout;
> +       unsigned long iicstat, timeout;
> +       int spins = 20;
>        int ret;
>
>        if (i2c->suspended)
> @@ -521,7 +522,22 @@ static int s3c24xx_i2c_doxfer(struct s3c24xx_i2c *i2c,
>
>        /* ensure the stop has been through the bus */
>
> -       msleep(1);
> +       dev_dbg(i2c->dev, "waiting for bus idle\n");
> +
> +       /* first, try busy waiting briefly */
> +       spins = 0;
          ^^^^^^^^^^^^ ?
> +       do {
> +               iicstat = readl(i2c->regs + S3C2410_IICSTAT);
> +       } while ((iicstat & S3C2410_IICSTAT_START) && --spins);
> +
> +       /* if that timed out sleep */
> +       if (!spins) {
> +               msleep(1);
> +               iicstat = readl(i2c->regs + S3C2410_IICSTAT);
> +       }
> +
> +       if (iicstat & S3C2410_IICSTAT_START)
> +               dev_warn(i2c->dev, "timeout waiting for bus idle\n");