x86/ioapic: Refactor the delay logic in timer_irq_works()

timer_irq_works() is used to detects the timer IRQs. It calls mdelay(10) to
delay ten ticks and check whether the timer IRQ work or not.

mdelay() depends on the loops_per_jiffy which is set up in
calibrate_delay(), but the delay calibration depends on a working timer
interrupt, which causes a chicken and egg problem.

The correct solution is to set up the interrupt mode and making sure that
the timer interrupt is delivered correctly before invoking calibrate_delay().
That means that mdelay() cannot be used in timer_irq_works(). 

Provide helper functions to make a rough delay estimate which is good enough
to prove that the timer interrupt is working. Either use TSC or a simple
delay loop and assume that 4GHz is the maximum CPU frequency to base the
delay calculation on.

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: yinghai@kernel.org
Cc: bhe@redhat.com
Link: https://lkml.kernel.org/r/1505293975-26005-9-git-send-email-douly.fnst@cn.fujitsu.com

1 files changed, 43 insertions, 2 deletions

diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index 70e48aa6af98..f8f248749c56 100644
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/arch/x86/kernel/apic/io_apic.c
@@ -1585,6 +1585,43 @@ static int __init notimercheck(char *s)
 }
 __setup("no_timer_check", notimercheck);
 
+static void __init delay_with_tsc(void)
+{
+	unsigned long long start, now;
+	unsigned long end = jiffies + 4;
+
+	start = rdtsc();
+
+	/*
+	 * We don't know the TSC frequency yet, but waiting for
+	 * 40000000000/HZ TSC cycles is safe:
+	 * 4 GHz == 10 jiffies
+	 * 1 GHz == 40 jiffies
+	 */
+	do {
+		rep_nop();
+		now = rdtsc();
+	} while ((now - start) < 40000000000UL / HZ &&
+		time_before_eq(jiffies, end));
+}
+
+static void __init delay_without_tsc(void)
+{
+	unsigned long end = jiffies + 4;
+	int band = 1;
+
+	/*
+	 * We don't know any frequency yet, but waiting for
+	 * 40940000000/HZ cycles is safe:
+	 * 4 GHz == 10 jiffies
+	 * 1 GHz == 40 jiffies
+	 * 1 << 1 + 1 << 2 +...+ 1 << 11 = 4094
+	 */
+	do {
+		__delay(((1U << band++) * 10000000UL) / HZ);
+	} while (band < 12 && time_before_eq(jiffies, end));
+}
+
 /*
  * There is a nasty bug in some older SMP boards, their mptable lies
  * about the timer IRQ. We do the following to work around the situation:
@@ -1603,8 +1640,12 @@ static int __init timer_irq_works(void)
 
 	local_save_flags(flags);
 	local_irq_enable();
-	/* Let ten ticks pass... */
-	mdelay((10 * 1000) / HZ);
+
+	if (boot_cpu_has(X86_FEATURE_TSC))
+		delay_with_tsc();
+	else
+		delay_without_tsc();
+
 	local_irq_restore(flags);
 
 	/*