Re: [RFC PATCH] Adding RTC device (work in progress)

[Zhaoming Luo]

On 11/9/24 10:58 PM, Sergey Bugaev wrote:
> I know that RTC can be configured to either represent local time, or
> in UTC. MS Windows prefers the former, GNU/Linux (or rather systemd?)
> prefers the latter [6]. Would it maybe make sense for the driver to
> have an --option to *pretend* the RTC stores UTC time when it's
> actually storing local time, transparently converting the time on
> read/write? The driver would learn the local timezone the same way all
> processes do (tzset?).
>
> [6]: https://wiki.archlinux.org/title/System_time
I think we don't need to add an option to pretend the RTC stores UTC 
time when it's actually storing local time, because I think the Hurd 
expects RTC storing UTC time.As mentioned in [6], 'The standard used by 
the hardware clock (CMOS clock, the BIOS time) is set by the operating 
system'. I did the following experiments in the virtual machine:

I changed the timezone to Chongqing:
```
sudo rm /etc/localtime
sudo ln -s /usr/share/zoneinfo/Asia/Chongqing /etc/localtime
```
Running `date` to get the following output:
```
Wed Nov 20 19:55:58 CST 2024
```
When I reboot the virtual machine I saw a line of log saying `saving 
system time to CMOS clock`.

After the reboot I setup the rtc translator and used the following 
program to read rtc. The time I read is UTC time, and the timezone I got 
by running `date` is still Chongqing. Therefore, I think the RTC time is 
independent of timezone for the Hurd.

This program reads the /tmp/rtc. (Please ignore the coding style :P, it 
was written before I started being aware of GNU coding style)
```
#include <stdio.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <hurd/ioctl.h>

struct rtc_time {
        int tm_sec;
        int tm_min;
        int tm_hour;
        int tm_mday;
        int tm_mon;
        int tm_year;
        int tm_wday;
        int tm_yday;
        int tm_isdst;
};

#define RTC_RD_TIME _IOR('p', 0x09, struct rtc_time)
#define _IOT_rtc_time _IOT(_IOTS(int),9,0,0,0,0)

int main() {
    int fd = open("/tmp/rtc", O_RDONLY);
    if (fd == -1) {
        printf("Cannot open rtc!\n");
        return 1;
    }
    printf("The fd of rtc is %d\n", fd);

    struct rtc_time time;
    int err = ioctl(fd, RTC_RD_TIME, &time);
    if (err == -1) {
        printf ("ioctl error!\n");
        return 1;
    }
    printf("tm_sec  : %d\n", time.tm_sec  );
    printf("tm_min  : %d\n", time.tm_min  );
    printf("tm_hour : %d\n", time.tm_hour );
    printf("tm_mday : %d\n", time.tm_mday );
    printf("tm_mon  : %d\n", time.tm_mon  );
    printf("tm_year : %d\n", time.tm_year );
    printf("tm_wday : %d\n", time.tm_wday );
    printf("tm_yday : %d\n", time.tm_yday );
    printf("tm_isdst: %d\n", time.tm_isdst);

    return 0;
}

```


--
Zhaoming Luo