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kernel/time/timecompare.c
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/* * Copyright (C) 2009 Intel Corporation. * Author: Patrick Ohly <patrick.ohly@intel.com> * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */ #include <linux/timecompare.h> #include <linux/module.h> |
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#include <linux/slab.h> |
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#include <linux/math64.h> |
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#include <linux/kernel.h> |
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/* * fixed point arithmetic scale factor for skew * * Usually one would measure skew in ppb (parts per billion, 1e9), but * using a factor of 2 simplifies the math. */ #define TIMECOMPARE_SKEW_RESOLUTION (((s64)1)<<30) ktime_t timecompare_transform(struct timecompare *sync, u64 source_tstamp) { u64 nsec; nsec = source_tstamp + sync->offset; nsec += (s64)(source_tstamp - sync->last_update) * sync->skew / TIMECOMPARE_SKEW_RESOLUTION; return ns_to_ktime(nsec); } |
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EXPORT_SYMBOL_GPL(timecompare_transform); |
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int timecompare_offset(struct timecompare *sync, s64 *offset, u64 *source_tstamp) { u64 start_source = 0, end_source = 0; struct { s64 offset; s64 duration_target; } buffer[10], sample, *samples; int counter = 0, i; int used; int index; int num_samples = sync->num_samples; |
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if (num_samples > ARRAY_SIZE(buffer)) { |
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samples = kmalloc(sizeof(*samples) * num_samples, GFP_ATOMIC); if (!samples) { samples = buffer; |
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num_samples = ARRAY_SIZE(buffer); |
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} } else { samples = buffer; } /* run until we have enough valid samples, but do not try forever */ i = 0; counter = 0; while (1) { u64 ts; ktime_t start, end; start = sync->target(); ts = timecounter_read(sync->source); end = sync->target(); if (!i) start_source = ts; /* ignore negative durations */ sample.duration_target = ktime_to_ns(ktime_sub(end, start)); if (sample.duration_target >= 0) { /* * assume symetric delay to and from source: * average target time corresponds to measured * source time */ sample.offset = |
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(ktime_to_ns(end) + ktime_to_ns(start)) / 2 - |
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ts; /* simple insertion sort based on duration */ index = counter - 1; while (index >= 0) { if (samples[index].duration_target < sample.duration_target) break; samples[index + 1] = samples[index]; index--; } samples[index + 1] = sample; counter++; } i++; if (counter >= num_samples || i >= 100000) { end_source = ts; break; } } *source_tstamp = (end_source + start_source) / 2; /* remove outliers by only using 75% of the samples */ used = counter * 3 / 4; if (!used) used = counter; if (used) { /* calculate average */ s64 off = 0; for (index = 0; index < used; index++) off += samples[index].offset; *offset = div_s64(off, used); } if (samples && samples != buffer) kfree(samples); return used; } |
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EXPORT_SYMBOL_GPL(timecompare_offset); |
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void __timecompare_update(struct timecompare *sync, u64 source_tstamp) { s64 offset; u64 average_time; if (!timecompare_offset(sync, &offset, &average_time)) return; if (!sync->last_update) { sync->last_update = average_time; sync->offset = offset; sync->skew = 0; } else { s64 delta_nsec = average_time - sync->last_update; /* avoid division by negative or small deltas */ if (delta_nsec >= 10000) { s64 delta_offset_nsec = offset - sync->offset; s64 skew; /* delta_offset_nsec * TIMECOMPARE_SKEW_RESOLUTION / delta_nsec */ u64 divisor; /* div_s64() is limited to 32 bit divisor */ skew = delta_offset_nsec * TIMECOMPARE_SKEW_RESOLUTION; divisor = delta_nsec; while (unlikely(divisor >= ((s64)1) << 32)) { /* divide both by 2; beware, right shift of negative value has undefined behavior and can only be used for the positive divisor */ skew = div_s64(skew, 2); divisor >>= 1; } skew = div_s64(skew, divisor); /* * Calculate new overall skew as 4/16 the * old value and 12/16 the new one. This is * a rather arbitrary tradeoff between * only using the latest measurement (0/16 and * 16/16) and even more weight on past measurements. */ #define TIMECOMPARE_NEW_SKEW_PER_16 12 sync->skew = div_s64((16 - TIMECOMPARE_NEW_SKEW_PER_16) * sync->skew + TIMECOMPARE_NEW_SKEW_PER_16 * skew, 16); sync->last_update = average_time; sync->offset = offset; } } } |
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EXPORT_SYMBOL_GPL(__timecompare_update); |