drivers/autotune: fix calibration with low frequency timers

When a single timer tick last less than the adjustment step time
(i.e. < 500 ns), we end up with a zero adjustment value which either
leads to a spurious early shot detection due to a rounding error, or a
null gravity value at the end of a lengthy but useless calibration
process.

Make sure to adjust by at least one timer tick, and mitigate rounding
errors when checking for early shots when the timer frequency is lower
than 1e9 / ADJUSTMENT_STEP.

kernel/drivers/autotune/autotune.c

@@ -32,9 +32,17 @@ MODULE_LICENSE("GPL");
 /* Auto-tuning services for the Cobalt core clock. */
 
 #define SAMPLING_TIME	500000000UL
+#define ADJUSTMENT_STEP 500
 #define WARMUP_STEPS	3
 #define AUTOTUNE_STEPS  40
 
+#define progress(__tuner, __fmt, __args...)				\
+	do {								\
+		if (!(__tuner)->quiet)					\
+			printk(XENO_INFO "autotune(%s) " __fmt "\n",	\
+			       (__tuner)->name, ##__args);		\
+	} while (0)
+
 struct tuning_score {
 	int pmean;
 	int stddev;
@@ -435,13 +443,6 @@ static inline void build_score(struct gravity_tuner *tuner, int step)
 	tuner->nscores++;
 }
 
-#define progress(__tuner, __fmt, __args...)				\
-	do {								\
-		if (!(__tuner)->quiet)					\
-			printk(XENO_INFO "autotune(%s) " __fmt "\n",	\
-			       (__tuner)->name, ##__args);		\
-	} while (0)
-
 static int cmp_score_mean(const void *c, const void *r)
 {
 	const struct tuning_score *sc = c, *sr = r;
@@ -541,7 +542,8 @@ static int tune_gravity(struct gravity_tuner *tuner, int period)
 	orig_gravity = tuner->get_gravity(tuner);
 	tuner->set_gravity(tuner, 0);
 	tuner->nscores = 0;
-	adjust = xnclock_ns_to_ticks(&nkclock, 500); /* Gravity adjustment step */
+	/* Gravity adjustment step */
+	adjust = xnclock_ns_to_ticks(&nkclock, ADJUSTMENT_STEP) ?: 1;
 	gravity_limit = state->step;
 	progress(tuner, "warming up...");
 
@@ -579,12 +581,20 @@ static int tune_gravity(struct gravity_tuner *tuner, int period)
 			continue;
 		}
 
+		/*
+		 * We should not be early by more than the gravity
+		 * value minus one tick, to account for the rounding
+		 * error involved when the timer frequency is lower
+		 * than 1e9 / ADJUSTMENT_STEP.
+		 */
 		if (state->min_lat < 0) {
-			if (tuner->get_gravity(tuner) == 0) {
+			if (tuner->get_gravity(tuner) < -state->min_lat - 1) {
 				printk(XENO_WARNING
 				       "autotune(%s) failed with early shot (%Ld ns)\n",
 				       tuner->name,
-				       xnclock_ticks_to_ns(&nkclock, state->min_lat));
+				       xnclock_ticks_to_ns(&nkclock,
+						   -(tuner->get_gravity(tuner) +
+						     state->min_lat)));
 				ret = -EAGAIN;
 				goto fail;
 			}