RTC boot-time synchronization

[pbalduino]

Goal

Integrate RTC hardware clock reading at boot time to initialize system time.

Context

Although we can read CMOS time (src/kernel/timer/rtc.c:16), the kernel never uses it. We need proper RTC integration at boot.

Current State

• RTC driver exists and can read hardware clock
• Limine bootloader provides timestamp, but we have no fallback
• System time initialization happens without consulting RTC

What's Needed

1. Boot-time sync path: Read RTC at boot if Limine timestamp is absent
2. Write path: Implement clock_settime() persistence to hardware
3. Integration point: src/kernel/init/main.c should call rtc_sync_system_time()

Implementation

// src/kernel/timer/rtc.c
void rtc_sync_system_time(void) {
    struct rtc_time hw_time;
    time_t unix_time;
    
    // Read hardware clock
    if (rtc_read_time(&hw_time) < 0) {
        kprintf("RTC: Failed to read hardware clock\n");
        return;
    }
    
    // Convert to Unix timestamp
    unix_time = rtc_to_unix_time(&hw_time);
    
    // Update system time if Limine didn't provide one
    if (system_boot_time == 0) {
        system_boot_time = unix_time;
        kprintf("RTC: System time initialized from hardware clock\n");
    }
}

// Write system time back to RTC
int rtc_set_time_from_unix(time_t unix_time) {
    struct rtc_time hw_time;
    unix_to_rtc_time(unix_time, &hw_time);
    return rtc_write_time(&hw_time);
}

Files to Modify

• src/kernel/timer/rtc.c - Add sync and write functions
• src/kernel/init/main.c - Call rtc_sync_system_time() early in boot
• src/kernel/syscall/syscall.c - Update sys_clock_settime() to persist to RTC

Testing

• Boot without Limine timestamp, verify RTC fallback
• Set system time with clock_settime(), verify RTC update
• Reboot and verify time persists
• Test with invalid RTC values

Dependencies

• #286 - Kernel RTC driver (✅ complete)
• #239 - Kernel time syscalls (✅ complete)

Parent Issue

• #226 - RTC and time management