Re: [PATCH v4 02/11] timekeeping: Add function to convert realtime to base clock

On Wed, Feb 07 2024 at 11:38, lakshmi sowjanya d. wrote:

From: Lakshmi Sowjanya D <redacted>

Introduce an interface, ktime_real_to_base_clock() to convert realtime
to base clock.

Convert the base clock to the system clock using convert_base_to_cs() in
get_device_system_crosststamp().

Add the helper function timekeeping_clocksource_has_base(), to check
whether the current clocksource has the same base clock.

Neither ktime_real_to_base_clock() nor
timekeeping_clocksource_has_base() are used anywhere.

What's the point of having them in the first place?

Your changelog explains the WHAT but not the WHY....

+static bool convert_clock(u64 *val, u32 numerator, u32 denominator)
+{
+	u64 rem, res;
+
+	if (numerator == 0 || denominator == 0)
+		return false;

What's wrong with the usual (!numerator || !denominator) notation?

+
+	res = div64_u64_rem(*val, denominator, &rem) * numerator;
+	*val = res + div_u64(rem * numerator, denominator);
+	return true;
+}
+
+static bool convert_base_to_cs(struct system_counterval_t *scv)
+{
+	struct clocksource *cs = tk_core.timekeeper.tkr_mono.clock;
+	struct clocksource_base *base = cs->base;
+
+	/* The timestamp was taken from the time keeper clock source */
+	if (cs->id == scv->cs_id)
+		return true;
+
+	/* Check whether cs_id matches the base clock */
+	if (!base || base->id != scv->cs_id)
+		return false;
+
+	/* Avoid conversion to a less precise clock */
+	if (scv->nsecs && cs->freq_khz != 0 && base->freq_khz < cs->freq_khz) {
+		if (!convert_clock(&scv->cycles, cs->freq_khz, USEC_PER_SEC))
+			return false;
+	} else {
+		if (scv->nsecs) {
+			if (!convert_clock(&scv->cycles, base->freq_khz, USEC_PER_SEC))
+				return false;
+		}
+		if (!convert_clock(&scv->cycles, base->numerator, base->denominator))
+			return false;
+	}

The above logic makes my brain hurt.

It's a reaonable requirement that cs->freq must be != 0 when sc->base !=
NULL and then converting from nanoseconds can always use cs->freq no
matter what the value of the base frequency is. Even for the case where
the base frequency is larger than cs->freq because the double conversion
does not give you more precision, right?

+	scv->cycles += base->offset;

So the whole thing can be reduced to:

   nom = scv->nsecs ? cs->freq_khz : base->numerator;
   den = scv->nsecs ? USEC_PER_SEC : base->denominator;

   convert(&scv->cycles, nom, den);
   scv->cycles += base->offset;

Thanks,

        tglx