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

[Peter Maydell]

On 24 March 2018 at 19:24, Michael Davidsaver  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 
> ---
>  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();
> +
> +{
> +/* 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(, 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

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

[Michael Davidsaver]

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 
---
 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);
 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();
+
+{
+/* 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(, s->offset);
+
+s->wday_offset = (user_wday - now.tm_wday) % 7 + 1;
+}
 }
 
 static int ds1338_send(I2CSlave *i2c, uint8_t data)
-- 
2.11.0