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N8004R / N8004V / N6004R /  N904plus / N904Vplus / N904 / N904V / N804T3 / N804A3 /

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X6003/T3004 / T3008(지정포트 LAN 8포트) 

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N104T / N104-r3 / N5-r1 / WE-R1 / mini

[링크 : http://www.iptime.co.kr/~iptime/bbs/view.php?id=faq_setup&no=587] 

Timer-based delays

In 2012, ARM contributed a new udelay implementation allowing the system timer built into many ARMv7 CPUs to be used instead of a busy-wait loop.[8] Timer based delays are more robust on systems that use frequency scaling to dynamically adjust the processor's speed at runtime, as loops_per_jiffies values may not necessarily scale linearly. Also, since the timer frequency is known in advance, no calibration is needed at boot time.

One side effect of this change is that the BogoMIPS value will reflect the timer frequency, not the CPU's core frequency. Typically the timer frequency is much lower than the processor's maximum frequency, and some users may be surprised to see an unusually low BogoMIPS value when comparing against systems that use traditional busy-wait loops.

[링크 : http://en.wikipedia.org/wiki/BogoMips] 

ARM: 7452/1: delay: allow timer-based delay implementation to be selected

This patch allows a timer-based delay implementation to be selected by

switching the delay routines over to use get_cycles, which is

implemented in terms of read_current_timer. This further allows us to

skip the loop calibration and have a consistent delay function in the

face of core frequency scaling.

To avoid the pain of dealing with memory-mapped counters, this

implementation uses the co-processor interface to the architected timers

when they are available. The previous loop-based implementation is

kept around for CPUs without the architected timers and we retain both

the maximum delay (2ms) and the corresponding conversion factors for

determining the number of loops required for a given interval. Since the

indirection of the timer routines will only work when called from C,

the sa1100 sleep routines are modified to branch to the loop-based delay

functions directly.

[링크 : https://git.kernel.org/.../linux.git/commit/?id=d0a533b18235d36206b9b422efadb7cee444dfdb]