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

Le 26/08/2019 à 13:41, Michael Ellerman a écrit :

Christophe Leroy [off-list ref] writes:

quoted

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 <redacted>
---
  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
else.

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 ?

Christophe

cheers

quoted

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