x86/tsc: Allow TSC calibration without PIT

Zhang Rui reported that a Surface Pro 4 will fail to boot with
lapic=notscdeadline. Part of the problem is that that machine doesn't have
a PIT.

If, for some reason, the TSC init has to fall back to TSC calibration, it
relies on the PIT to be present.

Allow TSC calibration to reliably fall back to HPET.

The below results in an accurate TSC measurement when forced on a IVB:

  tsc: Unable to calibrate against PIT
  tsc: No reference (HPET/PMTIMER) available
  tsc: Unable to calibrate against PIT
  tsc: using HPET reference calibration
  tsc: Detected 2792.451 MHz processor
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: len.brown@intel.com
Cc: rui.zhang@intel.com
Link: https://lkml.kernel.org/r/20171222092243.333145937@infradead.org

arch/x86/include/asm/i8259.h

@@ -69,6 +69,11 @@ struct legacy_pic {
 extern struct legacy_pic *legacy_pic;
 extern struct legacy_pic null_legacy_pic;
 
+static inline bool has_legacy_pic(void)
+{
+	return legacy_pic != &null_legacy_pic;
+}
+
 static inline int nr_legacy_irqs(void)
 {
 	return legacy_pic->nr_legacy_irqs;

arch/x86/kernel/tsc.c

@@ -25,6 +25,7 @@
 #include <asm/geode.h>
 #include <asm/apic.h>
 #include <asm/intel-family.h>
+#include <asm/i8259.h>
 
 unsigned int __read_mostly cpu_khz;	/* TSC clocks / usec, not used here */
 EXPORT_SYMBOL(cpu_khz);
@@ -363,6 +364,20 @@ static unsigned long pit_calibrate_tsc(u32 latch, unsigned long ms, int loopmin)
 	unsigned long tscmin, tscmax;
 	int pitcnt;
 
+	if (!has_legacy_pic()) {
+		/*
+		 * Relies on tsc_early_delay_calibrate() to have given us semi
+		 * usable udelay(), wait for the same 50ms we would have with
+		 * the PIT loop below.
+		 */
+		udelay(10 * USEC_PER_MSEC);
+		udelay(10 * USEC_PER_MSEC);
+		udelay(10 * USEC_PER_MSEC);
+		udelay(10 * USEC_PER_MSEC);
+		udelay(10 * USEC_PER_MSEC);
+		return ULONG_MAX;
+	}
+
 	/* Set the Gate high, disable speaker */
 	outb((inb(0x61) & ~0x02) | 0x01, 0x61);
 
@@ -487,6 +502,9 @@ static unsigned long quick_pit_calibrate(void)
 	u64 tsc, delta;
 	unsigned long d1, d2;
 
+	if (!has_legacy_pic())
+		return 0;
+
 	/* Set the Gate high, disable speaker */
 	outb((inb(0x61) & ~0x02) | 0x01, 0x61);