Linux操作系统内核对RTC的编程详解 （5）

static inline unsigned long 
mktime (unsigned int year, unsigned int mon, 
unsigned int day, unsigned int hour, 
unsigned int min, unsigned int sec) 
{ 
if (0 >= (int) (mon -= 2)) { /* 1..12 -> 11,12,1..10 */ 
mon += 12; /* Puts Feb last since it has leap day */ 
year -= 1; 
} 

return ((( 
(unsigned long) (year/4 - year/100 + year/400 + 367*mon/12 + day) + 
year*365 - 719499 
)*24 + hour /* now have hours */ 
)*60 + min /* now have minutes */ 
)*60 + sec; /* finally seconds */ 
}

static int set_rtc_mmss(unsigned long nowtime) 
{ 
int retval = 0; 
int real_seconds, real_minutes, cmos_minutes; 
unsigned char save_control, save_freq_select; 

/* gets recalled with irq locally disabled */ 
spin_lock(&rtc_lock); 
save_control = CMOS_READ(RTC_CONTROL); /* tell the clock it's being set */ 
CMOS_WRITE((save_control|RTC_SET), RTC_CONTROL); 

save_freq_select = CMOS_READ(RTC_FREQ_SELECT); /* stop and reset prescaler */ 
CMOS_WRITE((save_freq_select|RTC_DIV_RESET2), RTC_FREQ_SELECT); 

cmos_minutes = CMOS_READ(RTC_MINUTES); 
if (!(save_control & RTC_DM_BINARY) || RTC_ALWAYS_BCD) 
BCD_TO_BIN(cmos_minutes); 

/* 
* since we're only adjusting minutes and seconds, 
* don't interfere with hour overflow. This avoids 
* messing with unknown time zones but requires your 
* RTC not to be off by more than 15 minutes 
*/ 
real_seconds = nowtime % 60; 
real_minutes = nowtime / 60; 
if (((abs(real_minutes - cmos_minutes) + 15)/30) & 1) 
real_minutes += 30; /* correct for half hour time zone */ 
real_minutes %= 60; 

if (abs(real_minutes - cmos_minutes) < 30) { 
if (!(save_control & RTC_DM_BINARY) || RTC_ALWAYS_BCD) { 
BIN_TO_BCD(real_seconds); 
BIN_TO_BCD(real_minutes); 
} 
CMOS_WRITE(real_seconds,RTC_SECONDS); 
CMOS_WRITE(real_minutes,RTC_MINUTES); 
} else { 
printk(KERN_WARNING 
"set_rtc_mmss: can't update from %d to %d\n", 
cmos_minutes, real_minutes); 
retval = -1; 
} 

/* The following flags have to be released exactly in this order, 
* otherwise the DS12887 (popular MC146818A clone with integrated 
* battery and quartz) will not reset the oscillator and will not 
* update precisely 500 ms later. You won't find this mentioned in 
* the Dallas Semiconductor data sheets, but who believes data 
* sheets anyway ... -- Markus Kuhn 
*/ 
CMOS_WRITE(save_control, RTC_CONTROL); 
CMOS_WRITE(save_freq_select, RTC_FREQ_SELECT); 
spin_unlock(&rtc_lock); 

return retval; 
}

/* last time the cmos clock got updated */ 
static long last_rtc_update;