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imx8: detect cpu lcycles() frequency

--- a/sys/src/9/imx8/clock.c

+++ b/sys/src/9/imx8/clock.c

@@ -29,6 +29,9 @@

 void

 clockinit(void)

 {

+	uvlong tstart, tend;

+	ulong t0, t1;

+

 	syswr(PMCR_EL0, 1<<6 | 7);

 	syswr(PMCNTENSET, 1<<31);

 	syswr(PMUSERENR_EL0, 1<<2);

@@ -41,8 +44,22 @@

 		freq = sysrd(CNTFRQ_EL0);

 		print("timer frequency %lld Hz\n", freq);

 	}

-	m->cpuhz = freq;

-	m->cpumhz = (freq + Mhz/2 - 1) / Mhz;

+	tstart = sysrd(CNTPCT_EL0);

+	do{

+		t0 = lcycles();

+	}while(sysrd(CNTPCT_EL0) == tstart);

+	tend = tstart + (freq/100);

+	do{

+		t1 = lcycles();

+	}while(sysrd(CNTPCT_EL0) < tend);

+	t1 -= t0;

+	m->cpuhz = 100 * t1;

+	m->cpumhz = (m->cpuhz + Mhz/2 - 1) / Mhz;

+

+	/*

+	 * we are using virtual counter register CNTVCT_EL0

+	 * instead of the performance counter in userspace.

+	 */

 	m->cyclefreq = freq;

 	intrenable(IRQcntpns, localclockintr, nil, BUSUNKNOWN, "clock");

--- a/sys/src/9/imx8/main.c

+++ b/sys/src/9/imx8/main.c

@@ -135,6 +135,12 @@

 }

 void

+cpuidprint(void)

+{

+	iprint("cpu%d: %dMHz ARM Cortex A53\n", m->machno, m->cpumhz);

+}

+

+void

 main(void)

 {

 	machinit();

@@ -143,6 +149,7 @@

 		fpuinit();

 		intrinit();

 		clockinit();

+		cpuidprint();

 		synccycles();

 		timersinit();

 		flushtlb();

@@ -162,6 +169,7 @@

 	fpuinit();

 	intrinit();

 	clockinit();

+	cpuidprint();

 	timersinit();

 	pageinit();

 	procinit0();