RE: [Qemu-devel] [PATCH v4 5/7] RTC:Add RTC update-ended interrupt support

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> -----Original Message-----
> From: Stefano Stabellini [mailto:stefano.stabellini@xxxxxxxxxxxxx]
> Sent: Wednesday, March 21, 2012 2:04 AM
> On Mon, 19 Mar 2012, Zhang, Yang Z wrote:
> > Use a timer to emulate update cycle. When update cycle ended and UIE is
> setting, then raise an interrupt. The timer runs only when UF or AF is cleared.
> The idea is that if the user requests the update-ended interrupt (UIE)
> we setup a timer to inject it at the right time into the guest and
> another timer to update the UIP bit, correct?
No, the timer runs whenever the UF and AF is cleared, not only UIE or AIE is set.

> But do we actually need the second timer? Why can't we just update the
> UIP bit whenever the user tries to read reg A, as we do when UIE is not set?
The purpose of using two timer is trying to keep the UF, AF and UIP synchronous. User can poll UIP to check UF and AF bit. If we use timer for UF/AF bit track and check UIP by another way, since the timer will be fired with delay, then the problem is encountered: the UIP is cleared, but due to the delay of timer, the UF/AF bit is not set. So we need to check them on a same level. Although we can update UF/AF when reading it, the logic is too complicated, especially for AF bit. 

> Also I am not sure it is a great idea to rename rtc_timer_update into
> periodic_timer_update in the same patch where you introduce this new
> logic.
Ok, it make sense.

best regards
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