[coreboot-gerrit] Change in coreboot[master]: arm64: Print a char to UART early in exception handler

[Julius Werner (Code Review)]

Hello Hung-Te Lin,

I'd like you to do a code review. Please visit

https://review.coreboot.org/c/coreboot/+/37465

to review the following change.

Change subject: arm64: Print a char to UART early in exception handler
......................................................................

arm64: Print a char to UART early in exception handler

Over time our printk() seems to acquire more and more features... which
is nice, but it also makes it a little less robust when something goes
wrong. If the wrong global is trampled by some buffer overflow, it
suddenly doesn't print anymore. It would be nice to have at least some
way to tell that we triggered a real exception in that case.

With this patch, arm64 exceptions will print a '!' straight to the UART
before trying any of the more fancy printk() stuff. It's not much but it
should tell the difference between an exception and a hang and hopefully
help someone dig in the right direction sooner. This violates loglevels
(which is part of the point), but presumably when you have a fatal
exception you shouldn't care about that anymore.

Change-Id: I3b08ab86beaee55263786011caa5588d93bbc720
Signed-off-by: Julius Werner 
---
M src/arch/arm64/armv8/exception.c
1 file changed, 8 insertions(+), 2 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/65/37465/1

diff --git a/src/arch/arm64/armv8/exception.c b/src/arch/arm64/armv8/exception.c
index ec8fc05..c11ce57 100644
--- a/src/arch/arm64/armv8/exception.c
+++ b/src/arch/arm64/armv8/exception.c
@@ -34,6 +34,7 @@
 #include <arch/exception.h>
 #include <arch/transition.h>
 #include <console/console.h>
+#include <console/uart.h>
 #include <arch/lib_helpers.h>

uint8_t exception_stack[1*KiB] __attribute__((aligned(16)));
@@ -131,8 +132,13 @@

static void print_exception_info(struct exc_state *state, uint64_t idx)
 {
-	if (idx < NUM_EXC_VIDS)
-		printk(BIOS_DEBUG, "exception %s\n", exception_names[idx]);
+	/* Poor man's sign of life in case printk() is shot. */
+	__uart_tx_byte('\r');
+	__uart_tx_byte('\n');
+	__uart_tx_byte('!');
+
+	printk(BIOS_DEBUG, "\nexception %s\n",
+	       idx < NUM_EXC_VIDS ? exception_names[idx] : "_unknown");

print_regs(state);
    /* Few words below SP in case we need state from a returned function. */