[PATCH v3 3/3] arm64: Early boot time stamps

[Pavel Tatashin]

Allow printk time stamps/sched_clock() to be available from the early
boot.

Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
---
 arch/arm64/kernel/setup.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index 4b0e1231625c..28126facc4ed 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c
@@ -40,6 +40,7 @@
 #include <linux/efi.h>
 #include <linux/psci.h>
 #include <linux/sched/task.h>
+#include <linux/sched_clock.h>
 #include <linux/mm.h>
 
 #include <asm/acpi.h>
@@ -279,8 +280,32 @@ arch_initcall(reserve_memblock_reserved_regions);
 
 u64 __cpu_logical_map[NR_CPUS] = { [0 ... NR_CPUS-1] = INVALID_HWID };
 
+/*
+ * Get time stamps available early in boot, useful to identify boot time issues
+ * from the early boot.
+ */
+static __init void sched_clock_early_init(void)
+{
+	u64 (*read_time)(void) = arch_counter_get_cntvct;
+	u64 freq = arch_timer_get_cntfrq();
+
+	/*
+	 * The arm64 boot protocol mandates that CNTFRQ_EL0 reflects
+	 * the timer frequency. To avoid breakage on misconfigured
+	 * systems, do not register the early sched_clock if the
+	 * programmed value if zero. Other random values will just
+	 * result in random output.
+	 */
+	if (!freq)
+		return;
+
+	sched_clock_register(read_time, ARCH_TIMER_NBITS, freq);
+}
+
 void __init setup_arch(char **cmdline_p)
 {
+	sched_clock_early_init();
+
 	init_mm.start_code = (unsigned long) _text;
 	init_mm.end_code   = (unsigned long) _etext;
 	init_mm.end_data   = (unsigned long) _edata;
-- 
2.20.1