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bcm64: fix wrong prescaler for generic timer on rpi4

the raspberry pi 4 uses 54 instead of 19.2 MHz crystal.
detect which frequency is used by reading OTP bootmode
register:

https://www.raspberrypi.org/documentation/hardware/raspberrypi/otpbits.md

Bit 1: sets the oscillator frequency to 19.2MHz

--- a/sys/src/9/bcm64/clock.c

+++ b/sys/src/9/bcm64/clock.c

@@ -121,14 +121,23 @@

 	syswr(CNTP_TVAL_EL0, ~0UL);

 	if(m->machno == 0){

+		int oscfreq;

+

 		syswr(CNTP_CTL_EL0, Imask);

 		*(u32int*)(ARMLOCAL + GPUirqroute) = 0;

-		/* input clock is 19.2Mhz crystal */

+		/* bit 1 from OTP bootmode register determines OSC frequency */

+		if(*((u32int*)(VIRTIO+0x20f000)) & (1<<1))

+			oscfreq = 19200000;

+		else

+			oscfreq = 54000000;

+

+		/* input clock to OSC */

 		*(u32int*)(ARMLOCAL + Localctl) = 0;

+

 		/* divide by (2^31/Prescaler) */

-		*(u32int*)(ARMLOCAL + Prescaler) = (((uvlong)SystimerFreq<<31)/19200000)&~1UL;

+		*(u32int*)(ARMLOCAL + Prescaler) = (((uvlong)SystimerFreq<<31)/oscfreq)&~1UL;

 	} else {

 		syswr(CNTP_CTL_EL0, Enable);

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

--- a/sys/src/9/bcm64/sysreg.h

+++ b/sys/src/9/bcm64/sysreg.h

@@ -31,6 +31,7 @@

 #define PMCCNTR_EL0			SYSREG(3,3,9,13,0)

 #define PMUSERENR_EL0			SYSREG(3,3,9,14,0)

+#define CNTPCT_EL0			SYSREG(3,3,14,0,1)

 #define CNTP_TVAL_EL0			SYSREG(3,3,14,2,0)

 #define CNTP_CTL_EL0			SYSREG(3,3,14,2,1)

 #define CNTP_CVAL_EL0			SYSREG(3,3,14,2,2)