linux-kernel - [PATCH] x86: make delay work in earlier stages

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Date:   Thu, 19 Jan 2017 12:47:30 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: [PATCH] x86: make delay work in earlier stages

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 internally __const_udelay which is broken while booting.

The per_cpu cpu_info.loops_per_jiffy 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, but it
works relatively good. From the original (mdelay in rest_init):
[    0.170039] delaying 100ms
[    0.170828] done
I get:
[    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@...e.cz>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: <x86@...nel.org>
---
 arch/x86/lib/delay.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index 073d1f1a620b..aca0988f0e14 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -156,13 +156,14 @@ EXPORT_SYMBOL(__delay);

 inline void __const_udelay(unsigned long xloops)
 {
+	unsigned long lpf = 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" (lpf * (HZ / 4)));

 	__delay(++xloops);
 }
-- 
2.11.0