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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>
This commit is contained in:
Mark Brown 2012-11-21 13:12:11 +09:00 committed by Wolfram Sang
parent c5d5474425
commit 31f313d9be
1 changed files with 16 additions and 4 deletions
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20
drivers/i2c/busses/i2c-s3c2410.c
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@ -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) {