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-rw-r--r--payloads/libpayload/arch/x86/timer.c16
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;
}