1 /*-
23  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
34  * clockcalib(clk, clkname):
35  * Return the frequency of the provided timer, as calibrated against the
36  * current best-available timecounter.
39 clockcalib(uint64_t (*clk)(void), const char *clkname)  in clockcalib()
54 	/*-  in clockcalib()
55 	 * The idea here is to compute a best-fit linear regression between  in clockcalib()
57 	 * that line multiplied by the frequency of the reference clock gives  in clockcalib()
58 	 * us the frequency we're looking for.  in clockcalib()
68 	 * measured the target clock frequency.  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()
84 	 * would introduce a multiple-comparisons problem (cf. sequential  in clockcalib()
93 	 * we measure later in order to reduce floating-point rounding errors.  in clockcalib()
97 	clk0 = clk();  in clockcalib()
98 	t0 = tc->tc_get_timecount(tc) & tc->tc_counter_mask;  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()
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()
162 					1000000.0 / tc->tc_frequency),  in clockcalib()
169 		 * resulting from this loop synchronizing with the 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()
175 		while (clk() < clk_delay)  in clockcalib()