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>

1 files 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) {