Re: [PATCH 1/5] Add functions producing system time given a backing counter value

Thomas Gleixner
2015-07-29T12:52:55Z

On Wed, 29 Jul 2015, Peter Zijlstra wrote:
> On Wed, Jul 29, 2015 at 12:19:06PM +0200, Thomas Gleixner wrote:
> > I don't know whether we need functionality to convert arbitrary
> > timestamps at all, but if we really need them then they are going to
> > be pretty simple and explicitely not precise for anything else than
> > clock monotonic raw. But that's a different story.
>
> I think we need that too, and agreed, given NTP anything other than
> MONO_RAW is going to be fuzzy at best.

Yes, but that's a trivial case.

> > +static u64 art_to_tsc(u64 cycles)
> > +{
> > + /* FIXME: This needs 128bit math to work proper */
> > + return tsc_adjust + (cycles * tsc_numerator) / tsc_denominator;
>
> Yeah, its really unfortunate its given as a divisor and not a shift.
> That said I think, at least on the initial hardware, its 2, so:
>
> return mul_u64_u32_shr(cycles, tsc_numerator, 1);
>
> Should do, given that TSC_ADJUST had better be 0.

I don't trust BIOS folks :)

+ u64 tmp, res = tsc_adjust;
+
+ res += (cycles / tsc_denominator) * tsc_numerator;
+ tmp = (cycles % tsc_denominator) * tsc_numerator;
+ res += tmp / tsc_denominator;
+ return res;

That's what I have in my final patch.

> > + do {
> > + seq = read_seqcount_begin(&tk_core.seq);
> > + /*
> > + * Verify that the correlated clocksoure is related to
> > + * the currently installed timekeeper clocksoure
> > + */
> > + if (tk->tkr_mono.clock != crs->related_cs)
> > + return -ENODEV;
> > +
> > + /*
> > + * Try to get a timestamp from the device.
> > + */
> > + ret = crt->get_ts(crt);
> > + if (ret)
> > + return ret;
> > +
> > + /*
> > + * Convert the timestamp to timekeeper clock cycles
> > + */
> > + cycles = crs->convert(crs, crt->system_ts);
> > +
> > + /* Convert to clock realtime */
> > + base = ktime_add(tk->tkr_mono.base, tk_core.timekeeper.offs_real);
> > + nsecs = timekeeping_convert_to_ns(&tk->tkr_mono, cycles);
> > + crt->system_real = ktime_add_ns(base, nsecs);
> > +
> > + /* Convert to clock raw monotonic */
> > + base = tk->tkr_raw.base;
> > + nsecs = timekeeping_convert_to_ns(&tk->tkr_raw, cycles);
> > + crt->system_raw = ktime_add_ns(base, nsecs);
> > +
> > + } while (read_seqcount_retry(&tk_core.seq, seq));
> > + return 0;
> > +}
>
> This is still fuzzy, right? The hardware ART timestamp could be from
> _before_ the NTP adjust; or the NTP adjust could happen while we do this
> conversion and we'll take the retry loop.

I read the timestamp pair in the loop, so its always consistent.

> Any other ART users (buffered ETH frames) the delay will only get
> bigger.

That's a different story and we really can only convert this to
monotonic raw.

Thanks,

tglx