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57 lines
1.9 KiB
C++
57 lines
1.9 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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enum { TIMER_MIN_TICKS_PER_MS = 1000ULL };
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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
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* "ticks_per_ms >= TIMER_MIN_TICKS_PER_MS" is true when calling this
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* 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:RESULT_T {
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HALF_WIDTH = (sizeof(RESULT_T) << 2),
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MSB_MASK = ((RESULT_T)~0UL) << HALF_WIDTH,
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LSB_MASK = ((RESULT_T)~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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