From 31f313d9bebfc17e48c787c8c36b38662b4134a1 Mon Sep 17 00:00:00 2001 From: Mark Brown Date: Wed, 21 Nov 2012 13:12:11 +0900 Subject: [PATCH] i2c: s3c2410: Remove recently introduced performance overheads 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 Acked-by: Olof Johansson Reviewed-by: Daniel Kurtz Reviewed-by: Doug Anderson Signed-off-by: Wolfram Sang --- drivers/i2c/busses/i2c-s3c2410.c | 20 ++++++++++++++++---- 1 file changed, 16 insertions(+), 4 deletions(-) diff --git a/drivers/i2c/busses/i2c-s3c2410.c b/drivers/i2c/busses/i2c-s3c2410.c index d784c76ae3e..e93e7d67277 100644 --- a/drivers/i2c/busses/i2c-s3c2410.c +++ b/drivers/i2c/busses/i2c-s3c2410.c @@ -554,6 +554,7 @@ 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 */ @@ -566,12 +567,23 @@ 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; iicstat = readl(i2c->regs + S3C2410_IICSTAT); + while ((iicstat & S3C2410_IICSTAT_START) && --spins) { + cpu_relax(); + iicstat = readl(i2c->regs + S3C2410_IICSTAT); + } + + /* + * 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. + */ delay = 1; while ((iicstat & S3C2410_IICSTAT_START) && ktime_us_delta(now, start) < S3C2410_IDLE_TIMEOUT) {