1 /*-
20  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
36  * current best-available timecounter.
54 	/*-  in clockcalib()
55 	 * The idea here is to compute a best-fit linear regression between  in clockcalib()
66 	 * on an ongoing basis, updating all five values after each new data  in clockcalib()
74 	 *    (variance(x) * variance(y) - covariance(x, y)^2)  in clockcalib()
75 	 *    ------------------------------------------------  in clockcalib()
76 	 *              covariance(x, y)^2 * (N - 2)  in clockcalib()
81 	 * Finally, we need to determine when to stop gathering data.  We  in clockcalib()
82 	 * can't simply stop as soon as the computed uncertainty estimate  in clockcalib()
84 	 * would introduce a multiple-comparisons problem (cf. sequential  in clockcalib()
85 	 * analysis in clinical trials).  Instead, we stop with N data points  in clockcalib()
86 	 * if the estimated uncertainty of the first k data points meets our  in clockcalib()
93 	 * we measure later in order to reduce floating-point rounding errors.  in clockcalib()
98 	t0 = tc->tc_get_timecount(tc) & tc->tc_counter_mask;  in clockcalib()
104 		/* Get a new data point. */  in clockcalib()
105 		clk1 = clk() - clk0;  in clockcalib()
106 		t1 = tc->tc_get_timecount(tc) & tc->tc_counter_mask;  in clockcalib()
108 			tadj += (uint64_t)tc->tc_counter_mask + 1;  in clockcalib()
110 		t1 += tadj - t0;  in clockcalib()
113 		if (t1 > tc->tc_frequency) {  in clockcalib()
114 			printf("Statistical %s calibration failed!  "  in clockcalib()
117 			printf("Falling back to slow %s calibration.\n",  in clockcalib()
119 			freq = (double)(tc->tc_frequency) * clk1 / t1;  in clockcalib()
127 		d1 = clk1 - mu_clk;  in clockcalib()
129 		d2 = d1 * (clk1 - mu_clk);  in clockcalib()
130 		va_clk += (d2 - va_clk) * inv_n;  in clockcalib()
133 		d1 = t1 - mu_t;  in clockcalib()
135 		d2 = d1 * (t1 - mu_t);  in clockcalib()
136 		va_t += (d2 - va_t) * inv_n;  in clockcalib()
139 		d2 = d1 * (clk1 - mu_clk);  in clockcalib()
140 		cva += (d2 - cva) * inv_n;  in clockcalib()
143 		 * Count low-uncertainty iterations.  This is a rearrangement  in clockcalib()
149 		if (TSC_UNCERTAINTY_SQR * (n - 2) * cva * cva >  in clockcalib()
150 		    (va_t + 4) * (va_clk + 4) - cva * cva)  in clockcalib()
157 			freq = (double)(tc->tc_frequency) * cva / va_t;  in clockcalib()
159 				printf("Statistical %s calibration took"  in clockcalib()
160 				    " %lu us and %lu data points\n",  in clockcalib()
162 					1000000.0 / tc->tc_frequency),  in clockcalib()
171 		 * O(1 / n) time here -- long enough to avoid aliasing, but  in clockcalib()
174 		clk_delay = clk() + (clk() - clk0) / (n * n);  in clockcalib()