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[tip:timers/core] x86/timer: Make delay() work during early bootup

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Commit-ID:  4c45c5167c9563b1a2eee3e2fe954621355e4ca8
Gitweb:     http://git.kernel.org/tip/4c45c5167c9563b1a2eee3e2fe954621355e4ca8
Author:     Jiri Slaby <jslaby@suse.cz>
AuthorDate: Thu, 19 Jan 2017 12:47:30 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Sun, 22 Jan 2017 10:03:12 +0100

x86/timer: Make delay() work during early bootup

When a panic happens during bootup, "Rebooting in X seconds.." is
shown, but reboot happens immediatelly. It is because panic() uses mdelay()
and mdelay() calls __const_udelay() immediately, which does not
work while booting.

The per_cpu cpu_info.loops_per_jiffy value is not initialized yet, so
__const_udelay() actually multiplies the number of loops by zero. This
results in __const_udelay() to delay the execution only by a nanosecond
or so.

So check whether cpu_info.loops_per_jiffy is zero and use
loops_per_jiffy in that case. mdelay() will not be so precise without
proper calibration, but it works relatively well.

Before:

  [    0.170039] delaying 100ms
  [    0.170828] done

After

  [    0.214042] delaying 100ms
  [    0.313974] done

I do not think the added check matters given we are about to spin the
processor in the next few hundred cycles.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170119114730.2670-1-jslaby@suse.cz
[ Minor edits. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/lib/delay.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index 073d1f1..a8e91ae 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -156,13 +156,13 @@ EXPORT_SYMBOL(__delay);
 
 inline void __const_udelay(unsigned long xloops)
 {
+	unsigned long lpj = this_cpu_read(cpu_info.loops_per_jiffy) ? : loops_per_jiffy;
 	int d0;
 
 	xloops *= 4;
 	asm("mull %%edx"
 		:"=d" (xloops), "=&a" (d0)
-		:"1" (xloops), "0"
-		(this_cpu_read(cpu_info.loops_per_jiffy) * (HZ/4)));
+		:"1" (xloops), "0" (lpj * (HZ / 4)));
 
 	__delay(++xloops);
 }

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[PATCH] x86: make delay work in earlier stages Jiri Slaby <jslaby@suse.cz> - 2017-01-19 13:10 +0100
  Re: [PATCH] x86: make delay work in earlier stages Andy Shevchenko <andy.shevchenko@gmail.com> - 2017-01-19 20:10 +0100
  [tip:timers/core] x86/timer: Make delay() during early bootup tip-bot for Jiri Slaby <tipbot@zytor.com> - 2017-01-20 15:50 +0100
    Re: [tip:timers/core] x86/timer: Make delay() during early bootup Jiri Slaby <jslaby@suse.cz> - 2017-01-21 08:40 +0100
  [tip:timers/core] x86/timer: Make delay() work during early bootup tip-bot for Jiri Slaby <tipbot@zytor.com> - 2017-01-22 10:20 +0100

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