diff options
| -rw-r--r-- | payloads/libpayload/arch/x86/timer.c | 16 |
1 files changed, 9 insertions, 7 deletions
diff --git a/payloads/libpayload/arch/x86/timer.c b/payloads/libpayload/arch/x86/timer.c index ae288eb94e..40e81c4ced 100644 --- a/payloads/libpayload/arch/x86/timer.c +++ b/payloads/libpayload/arch/x86/timer.c @@ -49,6 +49,8 @@ u32 cpu_khz; unsigned int get_cpu_speed(void) { unsigned long long start, end; + const uint32_t clock_rate = 1193182; // 1.193182 MHz + const uint16_t interval = (2 * clock_rate) / 1000; // 2 ms /* Set up the PPC port - disable the speaker, enable the T2 gate. */ outb((inb(0x61) & ~0x02) | 0x01, 0x61); @@ -56,9 +58,9 @@ unsigned int get_cpu_speed(void) /* Set the PIT to Mode 0, counter 2, word access. */ outb(0xB0, 0x43); - /* Load the counter with 0xffff. */ - outb(0xff, 0x42); - outb(0xff, 0x42); + /* Load the interval into the counter. */ + outb(interval & 0xff, 0x42); + outb((interval >> 8) & 0xff, 0x42); /* Read the number of ticks during the period. */ start = rdtsc(); @@ -66,11 +68,11 @@ unsigned int get_cpu_speed(void) end = rdtsc(); /* - * The clock rate is 1193180 Hz, the number of milliseconds for a - * period of 0xffff is 1193180 / (0xFFFF * 1000) or .0182. - * Multiply that by the number of measured clocks to get the kHz value. + * The number of milliseconds for a period is + * clock_rate / (interval * 1000). Multiply that by the number of + * measured clocks to get the kHz value. */ - cpu_khz = (unsigned int)((end - start) * 1193180U / (1000 * 0xffff)); + cpu_khz = (end - start) * clock_rate / (1000 * interval); return cpu_khz; } |