[PATCH] i2c: i2c-s3c2410: Remove recently introduced performance overheads
DORMANTno replies
From: Mark Brown <hidden>
Date: 2012-11-20 05:57:21
Subsystem:
arm/samsung s3c, s5p and exynos arm architectures, i2c subsystem, i2c subsystem host drivers, the rest · Maintainers:
Krzysztof Kozlowski, Peter Griffin, Andi Shyti, Linus Torvalds
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-yzvPICuk2AATkU/dhu1WVueM+bqZidxxQQ4Iyu8u01E@public.gmane.org> --- drivers/i2c/busses/i2c-s3c2410.c | 21 ++++++++++++++++----- 1 file changed, 16 insertions(+), 5 deletions(-)
diff --git a/drivers/i2c/busses/i2c-s3c2410.c b/drivers/i2c/busses/i2c-s3c2410.c
index 31f802b..9050821 100644
--- a/drivers/i2c/busses/i2c-s3c2410.c
+++ b/drivers/i2c/busses/i2c-s3c2410.c@@ -530,6 +530,7 @@ 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 */
@@ -542,12 +543,22 @@ 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; + do { + cpu_relax(); + iicstat = readl(i2c->regs + S3C2410_IICSTAT); + } while ((iicstat & S3C2410_IICSTAT_START) && --spins); + + /* + * 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. */ - iicstat = readl(i2c->regs + S3C2410_IICSTAT); delay = 1; while ((iicstat & S3C2410_IICSTAT_START) && ktime_us_delta(now, start) < S3C2410_IDLE_TIMEOUT) {
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1.7.10.4