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npcx: CEC: Add software debouncing

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If pulses shorter than the CEC specification allows are detected,
ignore the bus for a while. This avoids CPU stress if there is a
misbehaving device sending short pulses on the CEC bus.

Signed-off-by: Stefan Adolfsson <sadolfsson@chromium.org>

BUG=b:76467407
BRANCH=none
TEST=none
CQ-DEPEND=CL:1030228

Change-Id: I55819f9437a00799718e235c30f256508465bf4c
Reviewed-on: https://chromium-review.googlesource.com/1030229
Commit-Ready: Stefan Adolfsson <sadolfsson@chromium.org>
Tested-by: Stefan Adolfsson <sadolfsson@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
This commit is contained in:
Stefan Adolfsson
2018-04-24 14:53:02 +02:00
committed by chrome-bot
parent 394ede0e6e
commit edac0b1924
1 changed files with 29 additions and 0 deletions
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29
chip/npcx/cec.c
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@@ -89,6 +89,19 @@
#define DATA_HIGH(data) DATA_TIME(HIGH, data)
#define DATA_LOW(data) DATA_TIME(LOW, data)
/*
* Number of short pulses seen before the debounce logic goes into ignoring
* the bus for DEBOUNCE_WAIT_LONG instead of DEBOUNCE_WAIT_SHORT
*/
#define DEBOUNCE_CUTOFF 3
/* The limit how short a start-bit can be to trigger debounce logic */
#define DEBOUNCE_LIMIT APB1_TICKS(200)
/* The time we ignore the bus for the first three debounce cases */
#define DEBOUNCE_WAIT_SHORT APB1_TICKS(100)
/* The time we ignore the bus after the three initial debounce cases */
#define DEBOUNCE_WAIT_LONG APB1_TICKS(500)
/*
* The variance in timing we allow outside of the CEC specification for
* incoming signals. Our measurements aren't 100% accurate either, so this
@@ -142,6 +155,7 @@ enum cec_state {
CEC_STATE_INITIATOR_ACK_VERIFY,
CEC_STATE_FOLLOWER_START_LOW,
CEC_STATE_FOLLOWER_START_HIGH,
CEC_STATE_FOLLOWER_DEBOUNCE,
CEC_STATE_FOLLOWER_HEADER_INIT_LOW,
CEC_STATE_FOLLOWER_HEADER_INIT_HIGH,
CEC_STATE_FOLLOWER_HEADER_DEST_LOW,
@@ -203,6 +217,8 @@ struct cec_rx {
* pulse duration
*/
int low_time;
/* Number of too short pulses seen in a row */
int debounce_count;
};
/* Transfer buffer and states */
@@ -550,9 +566,18 @@ void enter_state(enum cec_state new_state)
timeout = CAP_START_LOW;
break;
case CEC_STATE_FOLLOWER_START_HIGH:
cec_rx.debounce_count = 0;
cap_edge = CAP_EDGE_FALLING;
timeout = CAP_START_HIGH;
break;
case CEC_STATE_FOLLOWER_DEBOUNCE:
if (cec_rx.debounce_count >= DEBOUNCE_CUTOFF) {
timeout = DEBOUNCE_WAIT_LONG;
} else {
timeout = DEBOUNCE_WAIT_SHORT;
cec_rx.debounce_count++;
}
break;
case CEC_STATE_FOLLOWER_HEADER_INIT_LOW:
case CEC_STATE_FOLLOWER_HEADER_DEST_LOW:
case CEC_STATE_FOLLOWER_EOM_LOW:
@@ -723,6 +748,7 @@ static void cec_event_timeout(void)
break;
case CEC_STATE_FOLLOWER_START_LOW:
case CEC_STATE_FOLLOWER_START_HIGH:
case CEC_STATE_FOLLOWER_DEBOUNCE:
case CEC_STATE_FOLLOWER_HEADER_INIT_LOW:
case CEC_STATE_FOLLOWER_HEADER_INIT_HIGH:
case CEC_STATE_FOLLOWER_HEADER_DEST_LOW:
@@ -765,6 +791,9 @@ static void cec_event_cap(void)
if (VALID_LOW(START_BIT, t)) {
cec_rx.low_time = t;
enter_state(CEC_STATE_FOLLOWER_START_HIGH);
} else if (t < DEBOUNCE_LIMIT) {
/* Wait a bit if start-pulses are really short */
enter_state(CEC_STATE_FOLLOWER_DEBOUNCE);
} else {
enter_state(CEC_STATE_IDLE);
}