i2c: fix bus recovery stop mode timing

The I2C specification states that tsu:sto for standard mode timing must
be at minimum 4us. Pictographically, this is:

SCL: ____/~~~~~~~~~
SDA: _________/~~~~
       ->|    |<- 4us minimum

We are currently waiting 2.5us between asserting SCL and SDA, which is
in violation of the standard. Adjust the timings to ensure that we meet
what is stipulated as the minimum timings to ensure that all devices
correctly interpret the STOP bus transition.

This is more important than trying to generate a square wave with even
duty cycle.

Signed-off-by: Russell King <[email protected]>
Signed-off-by: Wolfram Sang <[email protected]>

Author: Russell King

drivers/i2c/i2c-core-base.c

@@ -186,10 +186,11 @@ int i2c_generic_scl_recovery(struct i2c_adapter *adap)
 	 * If we can set SDA, we will always create a STOP to ensure additional
 	 * pulses will do no harm. This is achieved by letting SDA follow SCL
 	 * half a cycle later. Check the 'incomplete_write_byte' fault injector
-	 * for details.
+	 * for details. Note that we must honour tsu:sto, 4us, but lets use 5us
+	 * here for simplicity.
 	 */
 	bri->set_scl(adap, scl);
-	ndelay(RECOVERY_NDELAY / 2);
+	ndelay(RECOVERY_NDELAY);
 	if (bri->set_sda)
 		bri->set_sda(adap, scl);
 	ndelay(RECOVERY_NDELAY / 2);
@@ -211,7 +212,13 @@ int i2c_generic_scl_recovery(struct i2c_adapter *adap)
 		scl = !scl;
 		bri->set_scl(adap, scl);
 		/* Creating STOP again, see above */
-		ndelay(RECOVERY_NDELAY / 2);
+		if (scl)  {
+			/* Honour minimum tsu:sto */
+			ndelay(RECOVERY_NDELAY);
+		} else {
+			/* Honour minimum tf and thd:dat */
+			ndelay(RECOVERY_NDELAY / 2);
+		}
 		if (bri->set_sda)
 			bri->set_sda(adap, scl);
 		ndelay(RECOVERY_NDELAY / 2);