x86, tsc: Fallback to normal calibration if fast MSR calibration fails

If we cannot calibrate TSC via MSR based calibration
try_msr_calibrate_tsc() stores zero to fast_calibrate and returns that
to the caller. This value gets then propagated further to clockevents
code resulting division by zero oops like the one below:

 divide error: 0000 [#1] PREEMPT SMP
 Modules linked in:
 CPU: 0 PID: 1 Comm: swapper/0 Tainted: G        W    3.13.0+ #47
 task: ffff880075508000 ti: ffff880075506000 task.ti: ffff880075506000
 RIP: 0010:[<ffffffff810aec14>]  [<ffffffff810aec14>] clockevents_config.part.3+0x24/0xa0
 RSP: 0000:ffff880075507e58  EFLAGS: 00010246
 RAX: ffffffffffffffff RBX: ffff880079c0cd80 RCX: 0000000000000000
 RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffffffffffff
 RBP: ffff880075507e70 R08: 0000000000000001 R09: 00000000000000be
 R10: 00000000000000bd R11: 0000000000000003 R12: 000000000000b008
 R13: 0000000000000008 R14: 000000000000b010 R15: 0000000000000000
 FS:  0000000000000000(0000) GS:ffff880079c00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
 CR2: ffff880079fff000 CR3: 0000000001c0b000 CR4: 00000000001006f0
 Stack:
  ffff880079c0cd80 000000000000b008 0000000000000008 ffff880075507e88
  ffffffff810aecb0 ffff880079c0cd80 ffff880075507e98 ffffffff81030168
  ffff880075507ed8 ffffffff81d1104f 00000000000000c3 0000000000000000
 Call Trace:
  [<ffffffff810aecb0>] clockevents_config_and_register+0x20/0x30
  [<ffffffff81030168>] setup_APIC_timer+0xc8/0xd0
  [<ffffffff81d1104f>] setup_boot_APIC_clock+0x4cc/0x4d8
  [<ffffffff81d0f5de>] native_smp_prepare_cpus+0x3dd/0x3f0
  [<ffffffff81d02ee9>] kernel_init_freeable+0xc3/0x205
  [<ffffffff8177c910>] ? rest_init+0x90/0x90
  [<ffffffff8177c91e>] kernel_init+0xe/0x120
  [<ffffffff8178deec>] ret_from_fork+0x7c/0xb0
  [<ffffffff8177c910>] ? rest_init+0x90/0x90

Prevent this from happening by:
 1) Modifying try_msr_calibrate_tsc() to return calibration value or zero
    if it fails.
 2) Check this return value in native_calibrate_tsc() and in case of zero
    fallback to use normal non-MSR based calibration.

[mw: Added subject and changelog]
Reported-and-tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Bin Gao <bin.gao@linux.intel.com>
Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/r/1392810750-18660-1-git-send-email-mika.westerberg@linux.intel.comSigned-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/include/asm/tsc.h

@@ -66,6 +66,6 @@ extern void tsc_save_sched_clock_state(void);
 extern void tsc_restore_sched_clock_state(void);
 
 /* MSR based TSC calibration for Intel Atom SoC platforms */
-int try_msr_calibrate_tsc(unsigned long *fast_calibrate);
+unsigned long try_msr_calibrate_tsc(void);
 
 #endif /* _ASM_X86_TSC_H */

arch/x86/kernel/tsc.c

@@ -653,13 +653,10 @@ unsigned long native_calibrate_tsc(void)
 
 	/* Calibrate TSC using MSR for Intel Atom SoCs */
 	local_irq_save(flags);
-	i = try_msr_calibrate_tsc(&fast_calibrate);
+	fast_calibrate = try_msr_calibrate_tsc();
 	local_irq_restore(flags);
-	if (i >= 0) {
-		if (i == 0)
-			pr_warn("Fast TSC calibration using MSR failed\n");
+	if (fast_calibrate)
 		return fast_calibrate;
-	}
 
 	local_irq_save(flags);
 	fast_calibrate = quick_pit_calibrate();

arch/x86/kernel/tsc_msr.c

@@ -77,21 +77,18 @@ static int match_cpu(u8 family, u8 model)
 
 /*
  * Do MSR calibration only for known/supported CPUs.
- * Return values:
- * -1: CPU is unknown/unsupported for MSR based calibration
- *  0: CPU is known/supported, but calibration failed
- *  1: CPU is known/supported, and calibration succeeded
+ *
+ * Returns the calibration value or 0 if MSR calibration failed.
  */
-int try_msr_calibrate_tsc(unsigned long *fast_calibrate)
+unsigned long try_msr_calibrate_tsc(void)
 {
-	int cpu_index;
 	u32 lo, hi, ratio, freq_id, freq;
+	unsigned long res;
+	int cpu_index;
 
 	cpu_index = match_cpu(boot_cpu_data.x86, boot_cpu_data.x86_model);
 	if (cpu_index < 0)
-		return -1;
-
-	*fast_calibrate = 0;
+		return 0;
 
 	if (freq_desc_tables[cpu_index].msr_plat) {
 		rdmsr(MSR_PLATFORM_INFO, lo, hi);
@@ -103,7 +100,7 @@ int try_msr_calibrate_tsc(unsigned long *fast_calibrate)
 	pr_info("Maximum core-clock to bus-clock ratio: 0x%x\n", ratio);
 
 	if (!ratio)
-		return 0;
+		goto fail;
 
 	/* Get FSB FREQ ID */
 	rdmsr(MSR_FSB_FREQ, lo, hi);
@@ -112,16 +109,19 @@ int try_msr_calibrate_tsc(unsigned long *fast_calibrate)
 	pr_info("Resolved frequency ID: %u, frequency: %u KHz\n",
 				freq_id, freq);
 	if (!freq)
-		return 0;
+		goto fail;
 
 	/* TSC frequency = maximum resolved freq * maximum resolved bus ratio */
-	*fast_calibrate = freq * ratio;
-	pr_info("TSC runs at %lu KHz\n", *fast_calibrate);
+	res = freq * ratio;
+	pr_info("TSC runs at %lu KHz\n", res);
 
 #ifdef CONFIG_X86_LOCAL_APIC
 	lapic_timer_frequency = (freq * 1000) / HZ;
 	pr_info("lapic_timer_frequency = %d\n", lapic_timer_frequency);
 #endif
+	return res;
 
-	return 1;
+fail:
+	pr_warn("Fast TSC calibration using MSR failed\n");
+	return 0;
 }