On Friday 30 November 2007, Pavel Machek wrote:
quoted text
> > 
> > It's not an issue of accidental writes, it's an issue of there being
> > no other synchronization for setting those alarms.  Remember that both
> > RTC_WKALM_SET and RTC_ALM_SET ioctls can set that same alarm, and so
> > could a different userspace activity ...
> 
> We have 3 interfaces to one hardware resource. I do not think kernel
> should try to arbitrate it here. There's just one alarm clock with
> three interfaces.

Having three interfaces is bad enough ... ensuring that none of
them can ever be used safely would be stupid.


quoted text
> > As written, this allows one userspace activity to clobber another if
> > it does so explicitly, by first disabling the other one and then
> > setting its own alarm.  But the idea is to minimize "accidents" like
> > unintentionally clobbering an alarm set by someone else.
> 
> Well, I could not get it to work with this "avoid-clobber" feature.

I had earlier pointed out a different issue, whereby "oneshot"
semantics weren't consistently followed.  I've been working on
some patches to address that.  The ACPI bits still need work,
but I'll forward one part soonish.


quoted text
> > > If I remove "accidental alarm modify" logic, I can actually use rtc to
> > > wake up more than once per boot.
> > 
> > Evidently the alarm isn't being disabled then...

    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
That's the issue addressed by those patches.  (Specific to rtc-cmos,
not to RTCs on saner hardware.)


quoted text
> I think we should just remove the 'avoid-clobber' logic. If userland
> wants to somehow arbitrate access, it can.

Pray tell, *HOW* could it arbitrate?


quoted text
> Now, if we want to provide "nice" interface for timed sleep, I think
> we can. Actually, I'd like to use normal select() as such an
> interface,

Yeah, what ever happened to timerfd?  :)


quoted text
> and enable kernel to automatically detect when it can sleep 
> and when it has to wake up.

There's the "rtcwake" thing.  That got merged into util-linux-ng, but
I happened to notice its setup_alarm() is calling gmtime() instead
of localtime() ... my suspicion is that the uClibc version I was using
had some timezone bugs, or else there's some other bug lurking in the
vicinity of dual-bootiness.

Note that "wakealarm" is not intende to be a "timed sleep" interface
at all ... it's just to set the wakealarm.  Something else is in charge
of putting the system to sleep, such as "echo mem > /sys/power/state"
or some GUI thing hooked up to logic that knows about how to make frame
buffers recover.

- Dave
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