Re: [RFC v2] ARM VM System Specification

[Arnd Bergmann <arnd@arndb.de>]

On Wednesday 11 June 2014 12:33:30 Grant Likely wrote:
> On Tue, Jun 10, 2014 at 7:56 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > Il 10/06/2014 20:08, Peter Maydell ha scritto:
> >
> >> On 10 June 2014 18:04, Christopher Covington <cov@codeaurora.org> 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.
> 
> In practical terms, yes the VM needs to provide an RTC interface, but
> I don't think it needs to appear in this spec, even if the kernel
> accesses it directly. Portable images should use the UEFI service.

It sounds like it should be in the spec then, if we want people building
portable images to include the efi-rtc driver.

	Arnd