lm4: Ensure falling edges on outputs to edge-sensitive host inputs

The KBD_IRQ#, SMI#, and SCI# lines on the host are sensitive to
falling edges.  When generating an interrupt, wait long enough to make
sure the lines are high before pulling them low, so that it reliably
generates a falling edge.  This solves a problem where calling the
IRQ-generation function twice in rapid succession could cause two low
pulses without an intervening logic-high as seen by the host (and thus
not a falling edge as seen by the host).

This is most visible on the keyboard line, because it can generate
back-to-back events on multi-byte scan codes.  Once the keyboard
mailbox is full, the EC will never attempt to fill it, and thus it
also won't attempt to generate another keyboard IRQ.  And since the
host missed the IRQ, it doesn't know it needs to empty the mailbox.

It could theoretically happen for the other lines, so fix them now
just to be safe.

This change should be low-impact and free from side effects.  4 usec
is a very small additional delay.  Even 65 usec added delay for
SCI/SMI is small, given that SCI/SMI events are typically much less
frequent (if they're happening very frequently, something else is
tragically wrong with the system...)

BUG=chrome-os-partner:27222
BRANCH=rambi
TEST=Bang on the keyboard like a monkey.  Keyboard shouldn't get stuck.

Change-Id: Id4e6de793b1f007f713bac8aa195ddd78feeea3e
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/193173
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-by: Benson Leung <bleung@chromium.org>

1 files changed, 21 insertions, 2 deletions

diff --git a/chip/lm4/lpc.c b/chip/lm4/lpc.c
index aa15e1fbfe..eb1e53a802 100644
--- a/chip/lm4/lpc.c
+++ b/chip/lm4/lpc.c
@@ -86,9 +86,18 @@ static void wait_irq_sent(void)
 #ifdef CONFIG_KEYBOARD_IRQ_GPIO
 static void keyboard_irq_assert(void)
 {
-	/* Negative edge-triggered keyboard interrupt. */
+	/*
+	 * Enforce signal-high for long enough for the signal to be pulled high
+	 * by the external pullup resistor.  This ensures the host will see the
+	 * following falling edge, regardless of the line state before this
+	 * function call.
+	 */
+	gpio_set_level(CONFIG_KEYBOARD_IRQ_GPIO, 1);
+	udelay(4);
+	/* Generate a falling edge */
 	gpio_set_level(CONFIG_KEYBOARD_IRQ_GPIO, 0);
-	wait_irq_sent();
+	udelay(4);
+	/* Set signal high, now that we've generated the edge */
 	gpio_set_level(CONFIG_KEYBOARD_IRQ_GPIO, 1);
 }
 #else
@@ -139,8 +148,13 @@ static inline void keyboard_irq_assert(void)
  */
 static void lpc_generate_smi(void)
 {
+	/* Enforce signal-high for long enough to debounce high */
+	gpio_set_level(GPIO_PCH_SMI_L, 1);
+	udelay(65);
+	/* Generate a falling edge */
 	gpio_set_level(GPIO_PCH_SMI_L, 0);
 	udelay(65);
+	/* Set signal high, now that we've generated the edge */
 	gpio_set_level(GPIO_PCH_SMI_L, 1);
 
 	if (host_events & event_mask[LPC_HOST_EVENT_SMI])
@@ -154,8 +168,13 @@ static void lpc_generate_smi(void)
 static void lpc_generate_sci(void)
 {
 #ifdef CONFIG_SCI_GPIO
+	/* Enforce signal-high for long enough to debounce high */
+	gpio_set_level(CONFIG_SCI_GPIO, 1);
+	udelay(65);
+	/* Generate a falling edge */
 	gpio_set_level(CONFIG_SCI_GPIO, 0);
 	udelay(65);
+	/* Set signal high, now that we've generated the edge */
 	gpio_set_level(CONFIG_SCI_GPIO, 1);
 #else
 	LM4_LPC_LPCCTL |= LM4_LPC_SCI_START;