linux-kernel - [PATCH] powerpc/time: use feature fixup in __USE

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Date:   Mon, 26 Aug 2019 06:59:34 +0000 (UTC)
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH] powerpc/time: use feature fixup in __USE_RTC() instead of cpu
 feature.

sched_clock(), used by printk(), calls __USE_RTC() to know
whether to use realtime clock or timebase.

__USE_RTC() uses cpu_has_feature() which is initialised by
machine_init(). Before machine_init(), __USE_RTC() returns true,
leading to a program check exception on CPUs not having realtime
clock.

In order to be able to use printk() earlier, use feature fixup.
Feature fixups are applies in early_init(), enabling the use of
printk() earlier.

Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
---
 arch/powerpc/include/asm/time.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/time.h b/arch/powerpc/include/asm/time.h
index 54f4ec1f9fab..3455cb54c333 100644
--- a/arch/powerpc/include/asm/time.h
+++ b/arch/powerpc/include/asm/time.h
@@ -42,7 +42,14 @@ struct div_result {
 /* Accessor functions for the timebase (RTC on 601) registers. */
 /* If one day CONFIG_POWER is added just define __USE_RTC as 1 */
 #ifdef CONFIG_PPC_BOOK3S_32
-#define __USE_RTC()	(cpu_has_feature(CPU_FTR_USE_RTC))
+static inline bool __USE_RTC(void)
+{
+	asm_volatile_goto(ASM_FTR_IFCLR("nop;", "b %1;", %0) ::
+			  "i" (CPU_FTR_USE_RTC) :: l_use_rtc);
+	return false;
+l_use_rtc:
+	return true;
+}
 #else
 #define __USE_RTC()	0
 #endif
-- 
2.13.3