Re: [Qemu-devel] [PATCH v6 2/4] cadence_ttc: initial version of device model

[Paul Brook <paul@codesourcery.com>]

> On Tue, Feb 21, 2012 at 11:04 PM, Paul Brook <paul@codesourcery.com> wrote:
> >> > +static inline int64_t is_between(int64_t x, int64_t a, int64_t b)
> >> > +{
> >> > +    if (a < b) {
> >> > +        return x > a && x <= b;
> >> > +    }
> >> > +    return x < a && x >= b;
> >> > +}
> >> 
> >> This looks slightly odd -- should the boundary condition for whether
> >> a value equal to the max/min really change depending on :whether a
> >> or b is greater?
> 
> The function determines whether x is in-between a and b exclusive of
> a, inclusive of b, so it is consistent with itself in that regard.
> 
> > This is a ugly hack.  Instead of figuring out whether we have a count-up
> > or count-down timer the code checks for both, and have the "in_between"
> > function magically DTRT.  I haven't followed the paths through in enough
> > detail to figure out whether it gets all the corner cases right.
> 
> Is it really a "hack"?? For count up b will always be greater than a,
> and for count down the reverse. I suppose I could assert these
> conditions at the call site for peace of mind? The invocation from
> cadence_timer_run doesn't care whether it is count up of count down,
> it really does just only care if the match value is in-between the
> current timer value and the next timer value, which is exactly what
> this function determines.

When you explain it like this, it makes a more sense.  But this isn't 
immediately obvious from the code.  It took me at least a couple of readings 
to figure out what was going on. This is exactly the sort of thing that should 
be described in comments.  A function with a very generic name is used in a 
way that has fairly subtle implications.  There's a good chance someone[1] 
will come along in a few months/years, reuse this function and "fix" the 
wierdness at the same time.

Annother non-obvious detail is the way you handle overflow.  Specifically you 
check a range both plus and minus the wrap value before wrapping the final 
count.  This is certainly confusing/surprising when you first encounter it.  
Very large steps result in overlapping ranges, which triggers [in this case 
harmless] warning bells.

Thinking about that, I realised why I don't like the following line:

> +    s->reg_value = (uint32_t)((x + interval) % interval);

This assumes x > -interval, which is not always true.

Paul

[1] "someone" includes me.  After I've forgotten this obscure detail.