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authorDaniel Kurtz <djkurtz@chromium.org>2012-11-15 16:13:32 +0400
committerWolfram Sang <w.sang@pengutronix.de>2012-11-16 16:09:33 +0400
commitfe724bf9f023384eb14431c0e49ec46017ba8e30 (patch)
tree8d860dd007fdf966375260697a1f5a1e6ed5400b /drivers/i2c
parent0da2e7768b4c2b4dbbb148ebe1606b6b4698fca2 (diff)
downloadlinux-fe724bf9f023384eb14431c0e49ec46017ba8e30.tar.xz
i2c: s3c2410: use exponential back off while polling for bus idle
Usually, the i2c controller has finished emitting the i2c STOP before the driver reaches the bus idle polling loop. Optimize for this most common case by reading IICSTAT first and potentially skipping the loop. If the cpu is faster than the hardware, we wait for bus idle in a polling loop. However, since the duration of one iteration of the loop is dependent on cpu freq, and this i2c IP is used on many different systems, use a time based loop timeout (5 ms). We would like very low latencies to detect bus idle for the normal 'fast' case. However, if a device is slow to release the bus for some reason, it could hold off the STOP generation for up to several milliseconds. Rapidly polling for bus idle would seriously load the CPU while waiting for it to release the bus. So, use a partial exponential backoff as a compromise between idle detection latency and cpu load. Signed-off-by: Daniel Kurtz <djkurtz@chromium.org> Signed-off-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com> Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Diffstat (limited to 'drivers/i2c')
-rw-r--r--drivers/i2c/busses/i2c-s3c2410.c67
1 files changed, 47 insertions, 20 deletions
diff --git a/drivers/i2c/busses/i2c-s3c2410.c b/drivers/i2c/busses/i2c-s3c2410.c
index a44e2130d3e0..dd93d3d6510a 100644
--- a/drivers/i2c/busses/i2c-s3c2410.c
+++ b/drivers/i2c/busses/i2c-s3c2410.c
@@ -50,6 +50,9 @@
#define QUIRK_HDMIPHY (1 << 1)
#define QUIRK_NO_GPIO (1 << 2)
+/* Max time to wait for bus to become idle after a xfer (in us) */
+#define S3C2410_IDLE_TIMEOUT 5000
+
/* i2c controller state */
enum s3c24xx_i2c_state {
STATE_IDLE,
@@ -557,6 +560,48 @@ static int s3c24xx_i2c_set_master(struct s3c24xx_i2c *i2c)
return -ETIMEDOUT;
}
+/* s3c24xx_i2c_wait_idle
+ *
+ * wait for the i2c bus to become idle.
+*/
+
+static void s3c24xx_i2c_wait_idle(struct s3c24xx_i2c *i2c)
+{
+ unsigned long iicstat;
+ ktime_t start, now;
+ unsigned long delay;
+
+ /* ensure the stop has been through the bus */
+
+ dev_dbg(i2c->dev, "waiting for bus idle\n");
+
+ start = now = ktime_get();
+
+ /*
+ * Most of the time, the bus is already idle within a few usec of the
+ * 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.
+ */
+ iicstat = readl(i2c->regs + S3C2410_IICSTAT);
+ delay = 1;
+ while ((iicstat & S3C2410_IICSTAT_START) &&
+ ktime_us_delta(now, start) < S3C2410_IDLE_TIMEOUT) {
+ usleep_range(delay, 2 * delay);
+ if (delay < S3C2410_IDLE_TIMEOUT / 10)
+ delay <<= 1;
+ now = ktime_get();
+ iicstat = readl(i2c->regs + S3C2410_IICSTAT);
+ }
+
+ if (iicstat & S3C2410_IICSTAT_START)
+ dev_warn(i2c->dev, "timeout waiting for bus idle\n");
+}
+
/* s3c24xx_i2c_doxfer
*
* this starts an i2c transfer
@@ -565,8 +610,7 @@ static int s3c24xx_i2c_set_master(struct s3c24xx_i2c *i2c)
static int s3c24xx_i2c_doxfer(struct s3c24xx_i2c *i2c,
struct i2c_msg *msgs, int num)
{
- unsigned long iicstat, timeout;
- int spins = 20;
+ unsigned long timeout;
int ret;
if (i2c->suspended)
@@ -604,24 +648,7 @@ static int s3c24xx_i2c_doxfer(struct s3c24xx_i2c *i2c,
if (i2c->quirks & QUIRK_HDMIPHY)
goto out;
- /* ensure the stop has been through the bus */
-
- dev_dbg(i2c->dev, "waiting for bus idle\n");
-
- /* first, try busy waiting briefly */
- do {
- cpu_relax();
- iicstat = readl(i2c->regs + S3C2410_IICSTAT);
- } while ((iicstat & S3C2410_IICSTAT_START) && --spins);
-
- /* if that timed out sleep */
- if (!spins) {
- msleep(1);
- iicstat = readl(i2c->regs + S3C2410_IICSTAT);
- }
-
- if (iicstat & S3C2410_IICSTAT_START)
- dev_warn(i2c->dev, "timeout waiting for bus idle\n");
+ s3c24xx_i2c_wait_idle(i2c);
out:
return ret;