Re: Delays, clocks, timers, hrtimers, etc

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Stefan Agner wrote:

On 2015-02-06 22:17, Mason wrote:

Do you also use the ARM local timers in your port?
Is there generic code to handle them?

It seems that there has been support for local timers once, but has been
removed. But I'm not aware of the details:

The equivalent gmane link would be:

The description used to say:

-	bool "Use local timer interrupts"
-	depends on SMP
-	default y
-	help
-	  Enable support for local timers on SMP platforms, rather then the
-	  legacy IPI broadcast method.  Local timers allows the system
-	  accounting to be spread across the timer interval, preventing a
-	  "thundering herd" at every timer tick.

which seems to have been replaced with HAVE_ARM_TWD

TWD stands for "Timer Watch Dog".

"ARM 11MP, Cortex-A5 and Cortex-A9 are often associated with a per-core
Timer-Watchdog (aka TWD), which provides both a per-cpu local timer
and watchdog.

The TWD is usually attached to a GIC to deliver its two per-processor

  depends on SMP
  select CLKSRC_OF if OF
    This options enables support for the ARM timer and watchdog unit

One problem I see is that HAVE_ARM_TWD depends on SMP...

One of the systems I want to support is UP (single-core Cortex A9).
Does that mean I should use an SMP kernel even for that system?
Or is there a different subsystem for UP systems?

Also, reading arch/arm/kernel/smp_twd.c, I see that frequency changes
due to cpufreq are properly accounted for. (Although I imagine they do
introduce a small error in time-keeping because moving from freq_A to
freq_B is not instantaneous, so the exact time elapsed in-between is
impossible to determine.)


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