Snow:Recover from stray pulses on i2c battery line

The I2C peripheral on the EC can get confused if there is a very
specific kind of noise introduced to the line.  This can be manifested
by jiggling the battery jack.  It gets the I2C into a state where
everything seems fine outwardly, but the device refuses to even transmit
START bits on the line.  It appears that one of the stray pulses on the
i2c bus gets the device off set from the actual bytes, leaving it
misinterpreting everything and waiting forever.  In this case, there is
only one way to recover (as you can't directly access these aspects of
the internal state) and that is to do a software reset of the i2c
peripheral.

Here I add some code to check for the condition where the EC was unable
to even send a START bit, and do a software reset of the i2c to recover.

BUG=chrome-os-partner:13161
TEST=With a faulty-battery-jack-board: Boot board, test that i2c works
by running "pmu" on the EC console.  Jiggle battery jack repeatedly
until errors are displayed on console.  Try to run pmu again.  Make sure
that it recovers gracefully, and do this many times.
BRANCH=snow

Change-Id: I91b8ef0c6f6079bc63f4a6a1bc91f67d19db9fc0
Signed-off-by: Charlie Mooney <charliemooney@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/32286
Reviewed-by: David Hendricks <dhendrix@chromium.org>

1 files changed, 18 insertions, 6 deletions

diff --git a/chip/stm32/i2c.c b/chip/stm32/i2c.c
index 160186a4a1..7a6d4fced3 100644
--- a/chip/stm32/i2c.c
+++ b/chip/stm32/i2c.c
@@ -801,12 +801,24 @@ static void handle_i2c_error(int port, int rv)
 	/* Clear busy state */
 	t1 = get_time();
 
-	/**
-	 * If the BUSY bit is faulty, send a stop bit just to be sure.  It
-	 * seems that this can be happen very briefly while sending a 1.
-	 * We've not actually seen this happen, but we just want to be safe.
-	 */
-	if (rv == EC_ERROR_TIMEOUT && !(r & 2)) {
+	if (rv == EC_ERROR_TIMEOUT && (STM32_I2C_CR1(port) & (1 << 8))) {
+		/*
+		 * If it failed while just trying to send the start bit then
+		 * something is wrong with the internal state of the i2c,
+		 * (Probably a stray pulse on the line got it out of sync with
+		 * the actual bytes) so reset it.
+		 */
+		CPRINTF("Unable to send START, resetting i2c.\n");
+		STM32_I2C_CR1(I2C2) = 0x8000;
+		STM32_I2C_CR1(I2C2) = 0x0000;
+		i2c_init2();
+		goto cr_cleanup;
+	} else if (rv == EC_ERROR_TIMEOUT && !(r & 2)) {
+		/*
+		 * If the BUSY bit is faulty, send a stop bit just to be sure.
+		 * It seems that this can be happen very briefly while sending
+		 * a 1. We've not actually seen this, but just to be safe.
+		 */
 		CPRINTF("Bad BUSY bit detected.\n");
 		master_stop(port);
 	}