Re: [Qemu-devel] [PATCH 07/14] timer: ds1338 fix wday_offset handling

[Peter Maydell]

On 24 March 2018 at 19:24, Michael Davidsaver <address@hidden> wrote:
> Correctly handle different real weekday in
> guest and host timezones.
> Allow guest to use any day as start of week
> (day 1).  eg. Monday instead of Sunday.
>
> Signed-off-by: Michael Davidsaver <address@hidden>
> ---
>  hw/timer/ds1338.c | 24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)
>
> diff --git a/hw/timer/ds1338.c b/hw/timer/ds1338.c
> index 071c031563..a969b5c8ba 100644
> --- a/hw/timer/ds1338.c
> +++ b/hw/timer/ds1338.c
> @@ -108,7 +108,10 @@ static void capture_current_time(DS1338State *s)
>      } else {
>          ARRAY_FIELD_DP32(s->nvram, HOUR, HOUR24, to_bcd(now.tm_hour));
>      }
> -    s->nvram[R_WDAY] = (now.tm_wday + s->wday_offset) % 7 + 1;
> +    s->nvram[R_WDAY] = (now.tm_wday + s->wday_offset) % 7;
> +    if (s->nvram[R_WDAY] == 0) {
> +        s->nvram[R_WDAY] = 7;
> +    }
>      s->nvram[R_DATE] = to_bcd(now.tm_mday);
>      s->nvram[R_MONTH] = to_bcd(now.tm_mon + 1);
>      s->nvram[R_YEAR] = to_bcd(now.tm_year - 100);
> @@ -182,17 +185,22 @@ static void ds1338_update(DS1338State *s)
>      } else {
>          now.tm_hour = from_bcd(ARRAY_FIELD_EX32(s->nvram, HOUR, HOUR24));
>      }
> -    {
> -        /* The day field is supposed to contain a value in
> -               the range 1-7. Otherwise behavior is undefined.
> -             */
> -        int user_wday = (s->nvram[R_WDAY] & 7) - 1;
> -        s->wday_offset = (user_wday - now.tm_wday + 7) % 7;
> -    }
> +    now.tm_wday = from_bcd(s->nvram[R_WDAY]) - 1u;
>      now.tm_mday = from_bcd(s->nvram[R_DATE] & 0x3f);

You don't need to set now.tm_wday -- qemu_timedate_diff()
will not use that field in the structure you hand it.

>      now.tm_mon = from_bcd(s->nvram[R_MONTH] & 0x1f) - 1;
>      now.tm_year = from_bcd(s->nvram[R_YEAR]) + 100;
>      s->offset = qemu_timedate_diff(&now);
> +
> +    {
> +        /* Round trip to get real wday_offset based on time delta and
> +         * ref. timezone.
> +         * Race if midnight (in ref. timezone) happens here.
> +         */
> +        int user_wday = now.tm_wday;
> +        qemu_get_timedate(&now, s->offset);
> +
> +        s->wday_offset = (user_wday - now.tm_wday) % 7 + 1;
> +    }
>  }

Ah, this part is fixing the bug I pointed out in the earlier patch.
You should just fold that bug fix into it.

thanks
-- PMM