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Commit 31f313d9 authored by Mark Brown's avatar Mark Brown Committed by Wolfram Sang
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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: default avatarMark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: default avatarOlof Johansson <olof@lixom.net>
Reviewed-by: default avatarDaniel Kurtz <djkurtz@chromium.org>
Reviewed-by: default avatarDoug Anderson <dianders@chromium.org>
Signed-off-by: default avatarWolfram Sang <w.sang@pengutronix.de>
parent c5d54744

drivers/i2c/busses/i2c-s3c2410.c

+16 −4
Original line number Diff line number Diff line
@@ -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) {