frostbyte/src/rtc.c

#include "rtc.h"
#include "ports.h"
#include "string.h"

#define rtcPortIndex 0x70
#define rtcPortData  0x71

#define rtcRegSeconds  0x00
#define rtcRegMinutes  0x02
#define rtcRegHours    0x04
#define rtcRegDay      0x07
#define rtcRegMonth    0x08
#define rtcRegYear     0x09
#define rtcRegStatusA  0x0A
#define rtcRegStatusB  0x0B

#define rtcStatusAUip  0x80

#define rtcStatusBHourFormat24 0x02
#define rtcStatusBBinaryMode   0x04
#define rtcHourPmBit           0x80

/* Bounded like ata.c's ataPollTimeout -- a wedged or absent RTC (never
   observed on real hardware or under QEMU, but this driver has no way to
   independently confirm the chip exists the way ataInit's IDENTIFY does)
   returns false/0 instead of spinning forever with interrupts disabled. */
#define rtcUipTimeout 100000u
#define rtcReadAttempts 8u

#define rtcAssumedCentury 20

static uint8 cmosRead(uint8 reg)
{
    /* Bit 0x80 on the index port disables NMI for the duration of the
       read, the same convention the OSDev wiki's own RTC driver uses --
       harmless if the platform ignores it, correct if it does not. */
    outb(rtcPortIndex, (uint8)(reg | 0x80));
    ioWait();
    return inb(rtcPortData);
}

static bool waitWhileUpdateInProgress(void)
{
    for (uint32 i = 0; i < rtcUipTimeout; i++) {
        if ((cmosRead(rtcRegStatusA) & rtcStatusAUip) == 0)
            return true;
    }
    return false;
}

typedef struct RtcRaw {
    uint8 seconds;
    uint8 minutes;
    uint8 hours;
    uint8 day;
    uint8 month;
    uint8 year;
} RtcRaw;

static void readRawOnce(RtcRaw* out)
{
    out->seconds = cmosRead(rtcRegSeconds);
    out->minutes = cmosRead(rtcRegMinutes);
    out->hours = cmosRead(rtcRegHours);
    out->day = cmosRead(rtcRegDay);
    out->month = cmosRead(rtcRegMonth);
    out->year = cmosRead(rtcRegYear);
}

uint8 rtcBcdToBinary(uint8 bcd)
{
    return (uint8)(((bcd >> 4) & 0x0F) * 10 + (bcd & 0x0F));
}

bool rtcReadDateTime(RtcDateTime* out)
{
    RtcRaw a;
    RtcRaw b;
    bool agree = false;

    for (uint32 attempt = 0; attempt < rtcReadAttempts && !agree; attempt++) {
        if (!waitWhileUpdateInProgress())
            return false;
        readRawOnce(&a);
        if (!waitWhileUpdateInProgress())
            return false;
        readRawOnce(&b);
        agree = memcmp(&a, &b, sizeof(RtcRaw)) == 0;
    }
    if (!agree)
        return false;

    uint8 statusB = cmosRead(rtcRegStatusB);
    bool binaryMode = (statusB & rtcStatusBBinaryMode) != 0;
    bool is24Hour = (statusB & rtcStatusBHourFormat24) != 0;

    bool pm = (a.hours & rtcHourPmBit) != 0;
    uint8 rawHours = (uint8)(a.hours & (uint8)~rtcHourPmBit);

    uint8 seconds = binaryMode ? a.seconds : rtcBcdToBinary(a.seconds);
    uint8 minutes = binaryMode ? a.minutes : rtcBcdToBinary(a.minutes);
    uint8 hours = binaryMode ? rawHours : rtcBcdToBinary(rawHours);
    uint8 day = binaryMode ? a.day : rtcBcdToBinary(a.day);
    uint8 month = binaryMode ? a.month : rtcBcdToBinary(a.month);
    uint8 year = binaryMode ? a.year : rtcBcdToBinary(a.year);

    if (!is24Hour) {
        if (pm && hours != 12)
            hours = (uint8)(hours + 12);
        else if (!pm && hours == 12)
            hours = 0;
    }

    out->year = rtcAssumedCentury * 100 + year;
    out->month = month;
    out->day = day;
    out->hour = hours;
    out->minute = minutes;
    out->second = seconds;
    return true;
}

/* Howard Hinnant's days_from_civil (proleptic Gregorian calendar), see
   http://howardhinnant.github.io/date_algorithms.html -- converts a civil
   calendar date into a day count relative to 1970-01-01 with no lookup
   tables, correct for the whole int64 range this kernel's RtcDateTime.year
   field can carry, including leap years computed structurally (divisible
   by 4, not by 100 unless also by 400) rather than by a hand-rolled rule. */
static int64 daysFromCivil(int64 y, int64 m, int64 d)
{
    y -= (m <= 2) ? 1 : 0;
    int64 era = (y >= 0 ? y : y - 399) / 400;
    int64 yoe = y - era * 400;                                   /* [0, 399] */
    int64 doy = (153 * (m + (m > 2 ? -3 : 9)) + 2) / 5 + d - 1;   /* [0, 365] */
    int64 doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;            /* [0, 146096] */
    return era * 146097 + doe - 719468;
}

int64 rtcEpochSeconds(const RtcDateTime* dt)
{
    int64 days = daysFromCivil(dt->year, dt->month, dt->day);
    return days * 86400 + dt->hour * 3600 + dt->minute * 60 + dt->second;
}

int64 rtcNow(void)
{
    RtcDateTime dt;
    if (!rtcReadDateTime(&dt))
        return 0;
    return rtcEpochSeconds(&dt);
}

/* Howard Hinnant's civil_from_days, the exact inverse of daysFromCivil
   above (same source) -- given a day count relative to 1970-01-01, returns
   the proleptic Gregorian year/month/day it names. Structurally mirrors
   daysFromCivil's era/yoe/doy decomposition in reverse, so the two stay
   easy to cross-check against each other by inspection. */
static void civilFromDays(int64 z, int64* year, int64* month, int64* day)
{
    z += 719468;
    int64 era = (z >= 0 ? z : z - 146096) / 146097;
    int64 doe = z - era * 146097;                                         /* [0, 146096] */
    int64 yoe = (doe - doe / 1460 + doe / 36524 - doe / 146096) / 365;     /* [0, 399] */
    int64 y = yoe + era * 400;
    int64 doy = doe - (365 * yoe + yoe / 4 - yoe / 100);                   /* [0, 365] */
    int64 mp = (5 * doy + 2) / 153;                                       /* [0, 11] */
    int64 d = doy - (153 * mp + 2) / 5 + 1;                               /* [1, 31] */
    int64 m = mp + (mp < 10 ? 3 : -9);                                    /* [1, 12] */
    *year = y + (m <= 2 ? 1 : 0);
    *month = m;
    *day = d;
}

void rtcCivilFromEpoch(int64 epochSeconds, RtcDateTime* out)
{
    int64 days = epochSeconds / 86400;
    int64 secondsOfDay = epochSeconds % 86400;
    /* C's integer division truncates toward zero, not toward negative
       infinity -- for a negative epochSeconds that does not land exactly
       on a day boundary, that rounds `days` the wrong way (toward 1970)
       and leaves secondsOfDay negative. Both need shifting back by
       exactly one day/86400s together, never one without the other. */
    if (secondsOfDay < 0) {
        secondsOfDay += 86400;
        days -= 1;
    }

    civilFromDays(days, &out->year, &out->month, &out->day);
    out->hour = secondsOfDay / 3600;
    out->minute = (secondsOfDay % 3600) / 60;
    out->second = secondsOfDay % 60;
}