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Subject: Re: [RFC][PATCH] new timeofday core subsystem (v. A3)
From: john stultz <johnstul@us.ibm.com>
To: Matt Mackall <mpm@selenic.com>, Christoph Lameter <clameter@sgi.com>
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Date: Mon, 14 Mar 2005 11:43:21 -0800
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On Mon, 2005-03-14 at 11:29 -0800, Matt Mackall wrote:
> On Mon, Mar 14, 2005 at 10:42:45AM -0800, john stultz wrote:
> > 
> > > > +static inline cycle_t read_timesource(struct timesource_t* ts)
> > > > +{
> > > > +	switch (ts->type) {
> > > > +	case TIMESOURCE_MMIO_32:
> > > > +		return (cycle_t)readl(ts->mmio_ptr);
> > > > +	case TIMESOURCE_MMIO_64:
> > > > +		return (cycle_t)readq(ts->mmio_ptr);
> > > > +	case TIMESOURCE_CYCLES:
> > > > +		return (cycle_t)get_cycles();
> > > > +	default:/* case: TIMESOURCE_FUNCTION */
> > > > +		return ts->read_fnct();
> > > > +	}
> > > > +}
> > > 
> > > Wouldn't it be better to change read_fnct to take a timesource * and
> > > then change all the other guys to generic_timesource_<foo> helper
> > > functions? This does away with the switch and makes it trivial to add
> > > new generic sources. Change mmio_ptr to void *private.
> > 
> > Not sure if I totally understand this, but originally I just had a read
> > function, but to allow this framework to function w/ ia64 fsyscalls (and
> > likely other arches vsyscalls) we need to pass the raw mmio pointers.
> > Thus the timesource type and switch idea was taken from the time
> > interpolator code.
> 
> Well for vsyscall, we can leave the mmio_ptr and type. But in-kernel,
> I think we'd rather always call read_fnct with generic helpers than hit this
> switch every time.

Huh. So if I understand you properly, all timesources should have valid
read_fnct pointers that return the cycle value, however we'll still
preserve the type and mmio_ptr so fsyscall/vsyscall bits can use them
externally?

Hmm. I'm a little cautious, as I really want to make the vsyscall
gettimeofday and regular do_gettimeofday be a similar as possible to
avoid some of the bugs we've seen between different gettimeofday
implementations. However I'm not completely against the idea.

Christoph: Do you have any thoughts on this?


> > > > +	if (time_suspend_state != TIME_RUNNING) {
> > > > +		printk(KERN_INFO "timeofday_suspend_hook: ACK! called while we're suspended!");
> > > 
> > > Line length. Perhaps BUG_ON instead.
> > 
> > Eh, its not fatal to BUG_ON seems a bit harsh. I'll fix the line length
> > though. 
> 
> Well there's a trade-off here. If it's something that should never
> happen and you only printk, you may never get a failure report
> (especially at KERN_INFO). It's good to be accomodating of external
> errors, but catching internal should-never-happen errors is important.

Fair enough. 


> > > Excellent question. 
> > 
> > Indeed.  Currently jiffies is used as both a interrupt counter and a
> > time unit, and I'm trying make it just the former. If I emulate it then
> > it stops functioning as a interrupt counter, and if I don't then I'll
> > probably break assumptions about jiffies being a time unit. So I'm not
> > sure which is the easiest path to go until all the users of jiffies are
> > audited for intent. 
> 
> Post this as a separate thread. There are various thoughts floating
> around on this already.

I'm a little busy with other things today, but I'll try to stir up a
discussion on this soon. 

thanks
-john


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