arch/arm64/arch_timer: Fix possible overflow in multiplication

The value from raw_read_cntfrq_el0() could be large enough to cause
overflow when multiplied by USECS_PER_SEC. To prevent this, both
USECS_PER_SEC and tfreq can be reduced by dividing them by their GCD.

BUG=b:307790895
TEST=emerge-geralt coreboot
TEST=boot to kernel and check the timestamps from `cbmem`

Change-Id: I366667de05392913150414f0fa9058725be71c52
Signed-off-by: Yidi Lin <yidilin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78800
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/arm64/arch_timer.c

@@ -1,12 +1,29 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
-#include <timer.h>
 #include <arch/lib_helpers.h>
+#include <commonlib/bsd/gcd.h>
+#include <timer.h>
 
 void timer_monotonic_get(struct mono_time *mt)
 {
 	uint64_t tvalue = raw_read_cntpct_el0();
-	uint32_t tfreq  = raw_read_cntfrq_el0();
+	static uint32_t tfreq, mult;
-	long usecs = (tvalue * 1000000) / tfreq;
+	uint32_t div;
+
+	/*
+	 * The value from raw_read_cntfrq_el0() could be large enough to
+	 * cause overflow when multiplied by USECS_PER_SEC. To prevent this,
+	 * both USECS_PER_SEC. and tfreq can be reduced by dividing them by
+	 * their GCD.
+	 */
+	if (tfreq == 0) {
+		tfreq = raw_read_cntfrq_el0();
+		mult = USECS_PER_SEC;
+		div = gcd32(tfreq, mult);
+		tfreq /= div;
+		mult /= div;
+	}
+
+	long usecs = (tvalue * mult) / tfreq;
 	mono_time_set_usecs(mt, usecs);
 }