[POWERPC] Fix timekeeping on PowerPC 601

Recent changes to the timekeeping code broke support for the PowerPC 601
processor which doesn't have the usual timebase facility but a slightly
different thing called (yuck) the RTC.

This fixes it, boot tested on an old 601 based PowerMac 7200.

Signed-off-by: Benjamin Herrenschmidt <[e-mail]>
Signed-off-by: Paul Mackerras <[e-mail]>

arch/powerpc/kernel/time.c

···
 	struct cpu_purr_data *p = &__get_cpu_var(cpu_purr_data);
 
 	local_irq_save(flags);
-	p->tb = mftb();
+	p->tb = get_tb_or_rtc();
 	p->purr = mfspr(SPRN_PURR);
 	wmb();
 	p->initialized = 1;
···
  */
 void snapshot_timebase(void)
 {
-	__get_cpu_var(last_jiffy) = get_tb();
+	__get_cpu_var(last_jiffy) = get_tb_or_rtc();
 	snapshot_purr();
 }
 
···
 
 		write_seqlock(&xtime_lock);
 		tb_next_jiffy = tb_last_jiffy + tb_ticks_per_jiffy;
+		if (__USE_RTC() && tb_next_jiffy >= 1000000000)
+			tb_next_jiffy -= 1000000000;
 		if (per_cpu(last_jiffy, cpu) >= tb_next_jiffy) {
 			tb_last_jiffy = tb_next_jiffy;
 			do_timer(1);
···
 	tb_to_ns_scale = scale;
 	tb_to_ns_shift = shift;
 	/* Save the current timebase to pretty up CONFIG_PRINTK_TIME */
-	boot_tb = get_tb();
+	boot_tb = get_tb_or_rtc();
 
 	tm = get_boot_time();
 

include/asm-powerpc/time.h

···
 }
 #endif /* !CONFIG_PPC64 */
 
+static inline u64 get_tb_or_rtc(void)
+{
+	return __USE_RTC() ? get_rtc() : get_tb();
+}
+
 static inline void set_tb(unsigned int upper, unsigned int lower)
 {
 	mtspr(SPRN_TBWL, 0);