Re: [Qemu-devel] [PATCH v6 2/7] RTC: Update the RTC clock only when reading it

[Paolo Bonzini <pbonzini@redhat.com>]

Note: I haven't tested this series yet.  Thanks for finding the problem!

Il 17/05/2012 04:28, Zhang, Yang Z ha scritto:
> Use offset instead of timer to calculate guest rtc. Guest rtc is calculated by (base_rtc + guest_time_now - guest_time_last_update_rtc + offset).
> Base_rtc means the rtc value of last update. 
> Guest_time_now means the guest time that access happens.
> Guest_time_last_update means the guest time of last update rtc.
> Offset is used when divider reset happened or set bit is changed.
> 
> +static void rtc_set_offset(RTCState *s, bool running, bool div_reset)
> +{
> +    if (div_reset)
> +        s->offset = 500000;
> +    else if (!running)
> +        s->offset = s->old_guest_usec % USEC_PER_SEC;
> +}

This is probably clearer if you just inline it at the appropriate
places---especially because rtc_set_offset(s, 0, 0) is a no-op.

> -static void rtc_update_second(void *opaque)
> -{
> -    RTCState *s = opaque;
> -    int64_t delay;
> -
> -    /* if the oscillator is not in normal operation, we do not update */
> -    if ((s->cmos_data[RTC_REG_A] & 0x70) != 0x20) {
> -        s->next_second_time += get_ticks_per_sec();
> -        qemu_mod_timer(s->second_timer, s->next_second_time);
> -    } else {
> -        rtc_next_second(&s->current_tm);
> -
> -        if (!(s->cmos_data[RTC_REG_B] & REG_B_SET)) {
> -            /* update in progress bit */
> -            s->cmos_data[RTC_REG_A] |= REG_A_UIP;
> -        }
> -        /* should be 244 us = 8 / 32768 seconds, but currently the
> -           timers do not have the necessary resolution. */
> -        delay = (get_ticks_per_sec() * 1) / 100;
> -        if (delay < 1)
> -            delay = 1;
> -        qemu_mod_timer(s->second_timer2,
> -                       s->next_second_time + delay);
> -    }
> -}

This patch breaks REG_A_UIP.  You can squash patches 2 and 3 together to
avoid this problem.

Paolo