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

[Stefano Stabellini]

On Thu, 22 Mar 2012, Zhang, Yang Z wrote:
> > -----Original Message-----
> > From: Stefano Stabellini [mailto:address@hidden
> > 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. 

fair enough