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 <broonie@opensource.wolfsonmicro.com>
Acked-by: Olof Johansson <olof@lixom.net>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>

diff --git a/drivers/i2c/busses/i2c-s3c2410.c b/drivers/i2c/busses/i2c-s3c2410.c
index d784c76..e93e7d6 100644 (file)
--- 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) {