I've been using ClockCycles() from the context of the clock interrupt
handler since 2004.

Dennis Kellly wrote:
> ClockCycles() is listed in docs as "unsafe" to call from an interrupt 
> handler.  However, for interrupt latency tests, there seems to be no 
> "safe" call for a precise timestamp.
> 
> Also release notes for 6.2.1B list "Execution of ClockCycles() inside an 
> interrupt causes kernel crash (Ref# 14369)" as having been fixed, i.e. 
> it must have been a valid use case.
> 
> Why is it listed as unsafe?  Does it re-enable interrupts on exit?
> 
> Thanks.
> 
> 
> 
> _______________________________________________
> OSTech
> http://community.qnx.com/sf/go/post5816
> 

> ClockCycles() is listed in docs as "unsafe" to call from an interrupt handler.
>   However, for interrupt latency tests, there seems to be no "safe" call for a
>  precise timestamp.
> 
> Also release notes for 6.2.1B list "Execution of ClockCycles() inside an 
> interrupt causes kernel crash (Ref# 14369)" as having been fixed, i.e. it must
>  have been a valid use case.
> 
> Why is it listed as unsafe?  Does it re-enable interrupts on exit?
> 
> Thanks.
> 


It's safe on x86 as for the other platform I don't know

Looking at the implementations in
  /usr/include/<platform>/neutrino.h
it looks like there are interrupt-safe implementations of ClockCycles()
for all platforms except for the ARM (i.e., mips, ppc, sh, and x86).

I guess a function is listed as "unsafe" as long as we cannot guarantee 
that the implementation will be [interrupt]-safe on /all/ platforms...

Oh, and the bit about re-enabling interrupts would only be true for 
kernel calls -- and those exactly you must avoid by all means in an 
interrupt handler (the handler was itself invoked by the kernel; doing
a kernel-call would introduce a reentrancy we cannot handle at that 
point). The inline-assembly-implementations of ClockCycles on non-arm-
platforms all just read some special CPU registers and maybe do a tiny 
bit of calculation; so there's no need to call kernel there.

Hope this helps,
- Thomas

> -----Ursprüngliche Nachricht-----
> Von: Dennis Kelly 
> Gesendet: 14 March 2008 16:44
> An: ostech-core_os
> Betreff: ClockCycles() use in interrupt handlers
> 
> 
> ClockCycles() is listed in docs as "unsafe" to call from an 
> interrupt handler.  However, for interrupt latency tests, 
> there seems to be no "safe" call for a precise timestamp.
> 
> Also release notes for 6.2.1B list "Execution of 
> ClockCycles() inside an interrupt causes kernel crash (Ref# 
> 14369)" as having been fixed, i.e. it must have been a valid use case.
> 
> Why is it listed as unsafe?  Does it re-enable interrupts on exit?
> 
> Thanks.
> 
> 
> 
> _______________________________________________
> OSTech
> http://community.qnx.com/sf/go/post5816
>