Re: [Xen-devel] [RFC v2] ARM VM System Specification

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

Il 10/06/2014 20:08, Peter Maydell ha scritto:
> On 10 June 2014 18:04, Christopher Covington <cov@xxxxxxxxxxxxxx> wrote:
> > On 06/10/2014 10:42 AM, Peter Maydell wrote:
> > > I just noticed that this doesn't mandate that the platform
> > > provides an RTC. As I understand it, the UEFI spec mandates
> > > that there's an RTC (could somebody more familiar with UEFI
> > > than me confirm/deny that?) so we should probably put one here.
> > Pardon my ignorance, but what exactly disqualifies Generic Timer
> > implementations from being used as Real Time Clocks?
> So my naive view was that an RTC actually had to have
> support for dealing with real (wall) clock time, ie
> knowing it's 2014 and not 1970. The generic timers are
> just timers. Or am I wrong and UEFI doesn't really
> require that?
The real-time clock provides four UEFI runtime services (GetTime, 
SetTime, GetWakeupTime, SetWakeupTime).  The spec says that you can 
return EFI_DEVICE_ERROR from GetTime/SetTime if "the time could not be 
retrieved/set due to a hardware error", but I don't think this is enough 
to make these two optional.  By comparison, GetWakeupTime/SetWakeupTime 
can also return EFI_UNSUPPORTED.
So I agree that the RTC is required in UEFI.

Paolo

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