On Thu, Jul 05, 2018 at 11:19:52AM -0700, Michael Davidsaver wrote:
> instead of a read-modify-write, do direct translation
> of device registers to struct tm members.
> 
> This new ds1338_update() is the reverse of
> the existing capture_current_time().
> 
> Simplifies later handling of CENTURY bit in
> similar Dallas RTC chips.
> 
> 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 <mdavidsaver@gmail.com>
> ---
>  hw/timer/ds1338.c | 95 ++++++++++++++++++++++++++++---------------------------
>  1 file changed, 48 insertions(+), 47 deletions(-)
> 
> diff --git a/hw/timer/ds1338.c b/hw/timer/ds1338.c
> index 637a0f4246..0134ffd72a 100644
> --- a/hw/timer/ds1338.c
> +++ b/hw/timer/ds1338.c
> @@ -110,7 +110,10 @@ static void capture_current_time(DS1338State *s)
>          /* 24 hour mode. */
>          s->nvram[R_HOUR] = 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;
> +    }

This looks a bit odd - you've removed the + 1 here, but...

>      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);
> @@ -161,6 +164,46 @@ static int ds1338_recv(I2CSlave *i2c)
>      return res;
>  }
>  
> +/* call after guest writes to current time registers
> + * to re-compute our offset from host time.
> + */
> +static void ds1338_update(DS1338State *s)
> +{
> +
> +    struct tm now = {};
> +
> +    /* TODO: Implement CH (stop) bit?  */
> +    now.tm_sec = from_bcd(s->nvram[R_SEC] & 0x7f);
> +    now.tm_min = from_bcd(s->nvram[R_MIN] & 0x7f);
> +    if (ARRAY_FIELD_EX32(s->nvram, HOUR, SET12)) {
> +        /* 12 hour (1-12) */
> +        /* read and wrap 1-12 -> 0-11 */
> +        now.tm_hour = from_bcd(ARRAY_FIELD_EX32(s->nvram, HOUR, HOUR12)) % 12u;
> +        if (ARRAY_FIELD_EX32(s->nvram, HOUR, AMPM)) {
> +            now.tm_hour += 12;
> +        }
> +
> +    } else {
> +        now.tm_hour = from_bcd(ARRAY_FIELD_EX32(s->nvram, HOUR, HOUR24));
> +    }
> +    now.tm_wday = from_bcd(s->nvram[R_WDAY]) - 1u;

.. you still have the -1 here.  Am I missing something?

> +    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(&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;
> +    }
> +}
> +
>  static int ds1338_send(I2CSlave *i2c, uint8_t data)
>  {
>      DS1338State *s = DS1338(i2c);
> @@ -170,52 +213,7 @@ static int ds1338_send(I2CSlave *i2c, uint8_t data)
>          s->addr_byte = false;
>          return 0;
>      }
> -    if (s->ptr < 7) {
> -        /* Time register. */
> -        struct tm now;
> -        qemu_get_timedate(&now, s->offset);
> -        switch(s->ptr) {
> -        case R_SEC:
> -            /* TODO: Implement CH (stop) bit.  */
> -            now.tm_sec = from_bcd(data & 0x7f);
> -            break;
> -        case R_MIN:
> -            now.tm_min = from_bcd(data & 0x7f);
> -            break;
> -        case R_HOUR:
> -            if (FIELD_EX32(data, HOUR, SET12)) {
> -                /* 12 hour (1-12) */
> -                /* read and wrap 1-12 -> 0-11 */
> -                now.tm_hour = from_bcd(FIELD_EX32(data, HOUR, HOUR12)) % 12u;
> -                if (FIELD_EX32(data, HOUR, AMPM)) {
> -                    now.tm_hour += 12;
> -                }
> -
> -            } else {
> -                now.tm_hour = from_bcd(FIELD_EX32(data, HOUR, HOUR24));
> -            }
> -            break;
> -        case R_WDAY:
> -            {
> -                /* The day field is supposed to contain a value in
> -                   the range 1-7. Otherwise behavior is undefined.
> -                 */
> -                int user_wday = (data & 7) - 1;
> -                s->wday_offset = (user_wday - now.tm_wday + 7) % 7;
> -            }
> -            break;
> -        case R_DATE:
> -            now.tm_mday = from_bcd(data & 0x3f);
> -            break;
> -        case R_MONTH:
> -            now.tm_mon = from_bcd(data & 0x1f) - 1;
> -            break;
> -        case R_YEAR:
> -            now.tm_year = from_bcd(data) + 100;
> -            break;
> -        }
> -        s->offset = qemu_timedate_diff(&now);
> -    } else if (s->ptr == R_CTRL) {
> +    if (s->ptr == R_CTRL) {
>          /* Control register. */
>  
>          /* Ensure bits 2, 3 and 6 will read back as zero. */
> @@ -227,6 +225,9 @@ static int ds1338_send(I2CSlave *i2c, uint8_t data)
>  
>      }
>      s->nvram[s->ptr] = data;
> +    if (s->ptr <= R_YEAR) {
> +        ds1338_update(s);
> +    }
>      inc_regptr(s);
>      return 0;
>  }

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson