On Tue, Nov 20, 2012 at 1:57 PM, Mark Brown
<[email protected]> wrote:
> The changes in "i2c-s3c2410: use exponential back off while polling for
> bus idle" remove the initial busy wait for I2C transfers to complete and
> replace it with usleep_range() calls which will schedule.
>
> Since for older SoCs I2C transfers would usually complete within an
> extremely small number of CPU cycles there is a win from not having to
> schedule. This happens because on the older SoCs the cores run at a
> smaller multiple of the speeds that the I2C bus is operating at; on more
> modern SoCs the busy wait is less likely to be effective.
>
> Fix the issue by restoring the busy wait, reducing the number of spins
> from 20 to 3 which covers the overwhelming majority of I2C transfers on
> the SoCs where the busy wait is effective.
>
> Signed-off-by: Mark Brown <[email protected]>
> ---
> drivers/i2c/busses/i2c-s3c2410.c | 21 ++++++++++++++++-----
> 1 file changed, 16 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/i2c/busses/i2c-s3c2410.c
> b/drivers/i2c/busses/i2c-s3c2410.c
> index 31f802b..9050821 100644
> --- a/drivers/i2c/busses/i2c-s3c2410.c
> +++ b/drivers/i2c/busses/i2c-s3c2410.c
> @@ -530,6 +530,7 @@ static void s3c24xx_i2c_wait_idle(struct s3c24xx_i2c *i2c)
> unsigned long iicstat;
> ktime_t start, now;
> unsigned long delay;
> + int spins;
>
> /* ensure the stop has been through the bus */
>
> @@ -542,12 +543,22 @@ static void s3c24xx_i2c_wait_idle(struct s3c24xx_i2c
> *i2c)
> * end of a transaction. However, really slow i2c devices can stretch
> * the clock, delaying STOP generation.
> *
> - * As a compromise between idle detection latency for the normal, fast
> - * case, and system load in the slow device case, use an exponential
> - * back off in the polling loop, up to 1/10th of the total timeout,
> - * then continue to poll at a constant rate up to the timeout.
> + * On slower SoCs this typically happens within a very small number of
> + * instructions so busy wait briefly to avoid scheduling overhead.
> + */
> + spins = 3;
> + do {
> + cpu_relax();
> + iicstat = readl(i2c->regs + S3C2410_IICSTAT);
> + } while ((iicstat & S3C2410_IICSTAT_START) && --spins);
Can you avoid one cpu_relax() by reading IICSTAT first, and switch to
while { } instead of do { } while ()?
> +
> + /*
> + * If we do get an appreciable delay as a compromise between idle
> + * detection latency for the normal, fast case, and system load in the
> + * slow device case, use an exponential back off in the polling loop,
> + * up to 1/10th of the total timeout, then continue to poll at a
> + * constant rate up to the timeout.
> */
> - iicstat = readl(i2c->regs + S3C2410_IICSTAT);
> delay = 1;
> while ((iicstat & S3C2410_IICSTAT_START) &&
> ktime_us_delta(now, start) < S3C2410_IDLE_TIMEOUT) {
> --
> 1.7.10.4
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