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authorMark Brown <broonie@opensource.wolfsonmicro.com>2012-11-21 13:12:11 +0900
committerWolfram Sang <w.sang@pengutronix.de>2012-11-22 22:34:40 +0100
commit31f313d9bebfc17e48c787c8c36b38662b4134a1 (patch)
tree7fcd40d17bac586a95ae21ed9f54fa0e642b4711 /drivers/i2c
parentc5d5474425c4e7e291a98e739ea65f8acd0d8d5c (diff)
downloadblackbird-op-linux-31f313d9bebfc17e48c787c8c36b38662b4134a1.tar.gz
blackbird-op-linux-31f313d9bebfc17e48c787c8c36b38662b4134a1.zip
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>
Diffstat (limited to 'drivers/i2c')
-rw-r--r--drivers/i2c/busses/i2c-s3c2410.c20
1 files changed, 16 insertions, 4 deletions
diff --git a/drivers/i2c/busses/i2c-s3c2410.c b/drivers/i2c/busses/i2c-s3c2410.c
index d784c76ae3e8..e93e7d672773 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) {
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