[Skiboot] [PATCH/UNTESTED] Add xscom_ok() and lpc_ok() to check XSCOM and LPC usability

[Benjamin Herrenschmidt]

This primarily checks whether the caller already holds the corresponding
locks to avoid re-entrancy in some of the deep error path such as when
XSCOM itself triggers an error log. It will be extended in the case of
LPC to also handle known HW error states.

We use them to avoid queuing/polling in the BT driver and to discard
characters in the UART driver.

Note: This will not normally involve a loss of log to the UART as the
UART driver is also protected by the console suspend mechanism. So
this is a safety mechanism only.

This aims at fixing issues where the generation of error logs inside
the LPC or XSCOM drivers could cause a re-entrancy (via the BT interface)
causing deadlocks. Now, the error logs IPMI messages will be queued up
and delivered later on the next poll handler.

Signed-off-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>
---
 hw/bt.c         | 11 +++++++++--
 hw/lpc-uart.c   |  4 ++++
 hw/lpc.c        | 12 ++++++++++++
 hw/xscom.c      |  5 +++++
 include/lpc.h   |  6 ++++++
 include/xscom.h |  5 +++++
 6 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/hw/bt.c b/hw/bt.c
index 7bf1b2f..8bb44cd 100644
--- a/hw/bt.c
+++ b/hw/bt.c
@@ -350,7 +350,8 @@ static void print_debug_queue_info(void) {}
 
 static void bt_send_and_unlock(void)
 {
-	if (bt.state == BT_STATE_IDLE && !list_empty(&bt.msgq))
+	if (lpc_ok() &&
+	    bt.state == BT_STATE_IDLE && !list_empty(&bt.msgq))
 		bt_send_msg();
 
 	unlock(&bt.lock);
@@ -361,6 +362,10 @@ static void bt_poll(struct timer *t __unused, void *data __unused)
 {
 	uint8_t bt_ctrl;
 
+	/* Don't do anything if the LPC bus is offline */
+	if (!lpc_ok())
+		return;
+
 	/* If we can't get the lock assume someone else will notice
 	 * the new message and process it. */
 	lock(&bt.lock);
@@ -440,7 +445,9 @@ static int bt_add_ipmi_msg(struct ipmi_msg *ipmi_msg)
 
 void bt_irq(void)
 {
-	uint8_t ireg = bt_inb(BT_INTMASK);
+	uint8_t ireg;
+
+	ireg = bt_inb(BT_INTMASK);
 
 	bt.irq_ok = true;
 	if (ireg & BT_INTMASK_B2H_IRQ) {
diff --git a/hw/lpc-uart.c b/hw/lpc-uart.c
index 2e6114a..7c3190e 100644
--- a/hw/lpc-uart.c
+++ b/hw/lpc-uart.c
@@ -135,6 +135,10 @@ static size_t uart_con_write(const char *buf, size_t len)
 {
 	size_t written = 0;
 
+	/* If LPC bus is bad, we just swallow data */
+	if (!lpc_ok())
+		return written;
+
 	lock(&uart_lock);
 	while(written < len) {
 		if (tx_room == 0) {
diff --git a/hw/lpc.c b/hw/lpc.c
index 0db674f..b287020 100644
--- a/hw/lpc.c
+++ b/hw/lpc.c
@@ -500,3 +500,15 @@ void lpc_used_by_console(void)
 		unlock(&chip->lpc_lock);
 	}
 }
+
+bool lpc_ok(void)
+{
+	struct proc_chip *chip;
+
+	if (lpc_default_chip_id < 0)
+		return false;
+	if (!xscom_ok())
+		return false;
+	chip = get_chip(lpc_default_chip_id);
+	return !lock_held_by_me(&chip->lpc_lock);
+}
diff --git a/hw/xscom.c b/hw/xscom.c
index a3533be..07b9b94 100644
--- a/hw/xscom.c
+++ b/hw/xscom.c
@@ -531,3 +531,8 @@ void xscom_used_by_console(void)
 	lock(&xscom_lock);
 	unlock(&xscom_lock);
 }
+
+bool xscom_ok(void)
+{
+	return !lock_held_by_me(&xscom_lock);
+}
diff --git a/include/lpc.h b/include/lpc.h
index 5611c54..a0990fd 100644
--- a/include/lpc.h
+++ b/include/lpc.h
@@ -27,6 +27,12 @@ extern void lpc_init(void);
 /* Check for a default bus */
 extern bool lpc_present(void);
 
+/* Return of LPC is currently usable. This can be false if the caller
+ * currently holds a lock that would make it unsafe, or the LPC bus
+ * is known to be in some error condition (TBI).
+ */
+extern bool lpc_ok(void);
+
 /* Handle the interrupt from LPC source */
 extern void __attrconst lpc_interrupt(uint32_t chip_id);
 
diff --git a/include/xscom.h b/include/xscom.h
index b1ecaf5..a841261 100644
--- a/include/xscom.h
+++ b/include/xscom.h
@@ -173,6 +173,11 @@ extern void xscom_init(void);
 /* Mark XSCOM lock as being in console path */
 extern void xscom_used_by_console(void);
 
+/* Returns true if XSCOM can be used. Typically this returns false if
+ * the current CPU holds the XSCOM lock (to avoid re-entrancy from error path).
+ */
+extern bool xscom_ok(void);
+
 extern int64_t xscom_read_cfam_chipid(uint32_t partid, uint32_t *chip_id);
 
 #endif /* __XSCOM_H */