[Skiboot] [PATCH 1/3] uart: Drop console write data if BMC becomes unresponsive

[Vasant Hegde]

If BMC becomes unresponsive (ex: during BMC reboot) during console write
then we may get stuck in uart_wait_tx_room(). This will result in kernel
lockups and in some cases host becomes unresponsive.

This patch retries for `tx_room` for predefined number of time. If we don't
get space within that time it will return zero. This will result in
dropping console message.

Signed-off-by: Vasant Hegde <hegdevasant at linux.vnet.ibm.com>
---
 hw/lpc-uart.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/hw/lpc-uart.c b/hw/lpc-uart.c
index 979a617c3..ed56a4bdc 100644
--- a/hw/lpc-uart.c
+++ b/hw/lpc-uart.c
@@ -63,6 +63,8 @@ static void *mmio_uart_base;
 static int uart_console_policy = UART_CONSOLE_OPAL;
 static int lpc_irq = -1;
 
+#define UART_WRITE_MAX_RETRIES	0x100
+
 void uart_set_console_policy(int policy)
 {
 	uart_console_policy = policy;
@@ -106,6 +108,8 @@ static void uart_check_tx_room(void)
 
 static void uart_wait_tx_room(void)
 {
+	int retry_cnt = 0;
+
 	while (!tx_room) {
 		uart_check_tx_room();
 		if (!tx_room) {
@@ -113,6 +117,10 @@ static void uart_wait_tx_room(void)
 			do {
 				barrier();
 				uart_check_tx_room();
+
+				retry_cnt++;
+				if (retry_cnt > UART_WRITE_MAX_RETRIES)
+					return;
 			} while (!tx_room);
 			smt_medium();
 		}
-- 
2.21.1