Re: [PATCH] powerpc/time: use feature fixup in __USE_RTC() instead of cpu feature.

From: Christophe Leroy
Date: Mon Aug 26 2019 - 07:48:50 EST

Le 26/08/2019 Ã 13:41, Michael Ellerman a ÃcritÂ:
Christophe Leroy <christophe.leroy@xxxxxx> writes:
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

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@xxxxxx>
arch/powerpc/include/asm/time.h | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

The other option would be just to make this a compile time decision, eg.
add CONFIG_PPC_601 and use that to gate whether we use RTC.

Euh ... yes OK, why not. And that would help simplify many places in the code. I can propose something in that direction, but it will be a bigger change.

Given how many 601 users there are, maybe 1?, I think that would be a
simpler option and avoids complicating the code / binary for everyone

However this patch doesn't complicate things more than it was with cpu_has_feature() which does exactly the same but using static keys, does it ?



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 */
-#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;
+ return true;
#define __USE_RTC() 0