/* calibrate.c: default delay calibration * * Excised from init/main.c * Copyright (C) 1991, 1992 Linus Torvalds */ #include <linux/jiffies.h> #include <linux/delay.h> #include <linux/init.h> #include <linux/timex.h> #include <linux/smp.h> unsigned long lpj_fine; unsigned long preset_lpj; static int __init lpj_setup(char *str) { preset_lpj = simple_strtoul(str,NULL,0); return 1; } __setup("lpj=", lpj_setup); #ifdef ARCH_HAS_READ_CURRENT_TIMER /* This routine uses the read_current_timer() routine and gets the * loops per jiffy directly, instead of guessing it using delay(). * Also, this code tries to handle non-maskable asynchronous events * (like SMIs) */ #define DELAY_CALIBRATION_TICKS ((HZ < 100) ? 1 : (HZ/100)) #define MAX_DIRECT_CALIBRATION_RETRIES 5 static unsigned long __cpuinit calibrate_delay_direct(void) { unsigned long pre_start, start, post_start; unsigned long pre_end, end, post_end; unsigned long start_jiffies; unsigned long timer_rate_min, timer_rate_max; unsigned long good_timer_sum = 0; unsigned long good_timer_count = 0; int i; if (read_current_timer(&pre_start) < 0 ) return 0; /* * A simple loop like * while ( jiffies < start_jiffies+1) * start = read_current_timer(); * will not do. As we don't really know whether jiffy switch * happened first or timer_value was read first. And some asynchronous * event can happen between these two events introducing errors in lpj. * * So, we do * 1. pre_start <- When we are sure that jiffy switch hasn't happened * 2. check jiffy switch * 3. start <- timer value before or after jiffy switch * 4. post_start <- When we are sure that jiffy switch has happened * * Note, we don't know anything about order of 2 and 3. * Now, by looking at post_start and pre_start difference, we can * check whether any asynchronous event happened or not */ for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) { pre_start = 0; read_current_timer(&start); start_jiffies = jiffies; while (time_before_eq(jiffies, start_jiffies + 1)) { pre_start = start; read_current_timer(&start); } read_current_timer(&post_start); pre_end = 0; end = post_start; while (time_before_eq(jiffies, start_jiffies + 1 + DELAY_CALIBRATION_TICKS)) { pre_end = end; read_current_timer(&end); } read_current_timer(&post_end); timer_rate_max = (post_end - pre_start) / DELAY_CALIBRATION_TICKS; timer_rate_min = (pre_end - post_start) / DELAY_CALIBRATION_TICKS; /* * If the upper limit and lower limit of the timer_rate is * >= 12.5% apart, redo calibration. */ if (pre_start != 0 && pre_end != 0 && (timer_rate_max - timer_rate_min) < (timer_rate_max >> 3)) { good_timer_count++; good_timer_sum += timer_rate_max; } } if (good_timer_count) return (good_timer_sum/good_timer_count); printk(KERN_WARNING "calibrate_delay_direct() failed to get a good " "estimate for loops_per_jiffy.\nProbably due to long platform interrupts. Consider using \"lpj=\" boot option.\n"); return 0; } #else static unsigned long __cpuinit calibrate_delay_direct(void) {return 0;} #endif /* * This is the number of bits of precision for the loops_per_jiffy. Each * time we refine our estimate after the first takes 1.5/HZ seconds, so try * to start with a good estimate. * For the boot cpu we can skip the delay calibration and assign it a value * calculated based on the timer frequency. * For the rest of the CPUs we cannot assume that the timer frequency is same as * the cpu frequency, hence do the calibration for those. */ #define LPS_PREC 8 static unsigned long __cpuinit calibrate_delay_converge(void) { /* First stage - slowly accelerate to find initial bounds */ unsigned long lpj, lpj_base, ticks, loopadd, loopadd_base, chop_limit; int trials = 0, band = 0, trial_in_band = 0; lpj = (1<<12); /* wait for "start of" clock tick */ ticks = jiffies; while (ticks == jiffies) ; /* nothing */ /* Go .. */ ticks = jiffies; do { if (++trial_in_band == (1<<band)) { ++band; trial_in_band = 0; } __delay(lpj * band); trials += band; } while (ticks == jiffies); /* * We overshot, so retreat to a clear underestimate. Then estimate * the largest likely undershoot. This defines our chop bounds. */ trials -= band; loopadd_base = lpj * band; lpj_base = lpj * trials; recalibrate: lpj = lpj_base; loopadd = loopadd_base; /* * Do a binary approximation to get lpj set to * equal one clock (up to LPS_PREC bits) */ chop_limit = lpj >> LPS_PREC; while (loopadd > chop_limit) { lpj += loopadd; ticks = jiffies; while (ticks == jiffies) ; /* nothing */ ticks = jiffies; __delay(lpj); if (jiffies != ticks) /* longer than 1 tick */ lpj -= loopadd; loopadd >>= 1; } /* * If we incremented every single time possible, presume we've * massively underestimated initially, and retry with a higher * start, and larger range. (Only seen on x86_64, due to SMIs) */ if (lpj + loopadd * 2 == lpj_base + loopadd_base * 2) { lpj_base = lpj; loopadd_base <<= 2; goto recalibrate; } return lpj; } void __cpuinit calibrate_delay(void) { static bool printed; if (preset_lpj) { loops_per_jiffy = preset_lpj; if (!printed) pr_info("Calibrating delay loop (skipped) " "preset value.. "); } else if ((!printed) && lpj_fine) { loops_per_jiffy = lpj_fine; pr_info("Calibrating delay loop (skipped), " "value calculated using timer frequency.. "); } else if ((loops_per_jiffy = calibrate_delay_direct()) != 0) { if (!printed) pr_info("Calibrating delay using timer " "specific routine.. "); } else { if (!printed) pr_info("Calibrating delay loop... "); loops_per_jiffy = calibrate_delay_converge(); } if (!printed) pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n", loops_per_jiffy/(500000/HZ), (loops_per_jiffy/(5000/HZ)) % 100, loops_per_jiffy); printed = true; }