399e1586be
There are hardware timers whose frequency can't be expressed as ticks-per-microsecond integer-value because only a ticks-per-millisecond integer-value is precise enough. We don't want to use expensive floating-point values here but nonetheless want to translate from ticks to time with microseconds precision. Thus, we split the input in two and translate both parts separately. This way, we can raise precision by shifting the values to their optimal bit position. Afterwards, the results are shifted back and merged together again. As this algorithm is not so trivial anymore and used by at least three timer drivers (base-hw/x86_64, base-hw/cortex_a9, timer/pit), move it to a generic header to avoid redundancy. Ref #2400
54 lines
1.8 KiB
C++
54 lines
1.8 KiB
C++
/*
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* \brief Utilities for timer drivers
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* \author Martin Stein
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* \date 2017-08-23
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*/
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/*
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* Copyright (C) 2017 Genode Labs GmbH
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*
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* This file is part of the Genode OS framework, which is distributed
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* under the terms of the GNU Affero General Public License version 3.
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*/
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#ifndef _DRIVERS__TIMER__UTIL_H_
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#define _DRIVERS__TIMER__UTIL_H_
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namespace Genode {
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/*
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* Translate timer ticks to microseconds without losing precicision
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*
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* There are hardware timers whose frequency can't be expressed as
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* ticks-per-microsecond integer-value because only a ticks-per-millisecond
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* integer-value is precise enough. We don't want to use expensive
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* floating-point values here but nonetheless want to translate from ticks
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* to time with microseconds precision. Thus, we split the input in two and
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* translate both parts separately. This way, we can raise precision by
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* shifting the values to their optimal bit position. Afterwards, the
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* results are shifted back and merged together again.
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*
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* Please ensure that the assertion "ticks_per_ms >= 1000" is true
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* when calling this method!
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*/
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template <typename RESULT_T, typename TICS_PER_MS_T>
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RESULT_T timer_ticks_to_us(RESULT_T const ticks,
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TICS_PER_MS_T const ticks_per_ms)
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{
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enum {
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HALF_WIDTH = (sizeof(RESULT_T) << 2),
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MSB_MASK = ~0UL << HALF_WIDTH,
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LSB_MASK = ~0UL >> HALF_WIDTH,
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MSB_RSHIFT = 10,
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LSB_LSHIFT = HALF_WIDTH - MSB_RSHIFT,
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};
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RESULT_T const msb = ((((ticks & MSB_MASK) >> MSB_RSHIFT)
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* 1000) / ticks_per_ms) << MSB_RSHIFT;
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RESULT_T const lsb = ((((ticks & LSB_MASK) << LSB_LSHIFT)
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* 1000) / ticks_per_ms) >> LSB_LSHIFT;
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return msb + lsb;
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}
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}
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#endif /* _DRIVERS__TIMER__UTIL_H_ */
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