[Asterisk-Dev] what's the value of a jiffie in zaptel driver ?

[Derek Smithies]

Hi,
  In the 2.4 series there are a bundle of  macros, 
time_after, 
time_before, 
time_after_eq, 
time_before_eq

from include/linux/timer.h

In 2.6.9, these marcos are in include/linux/jiffies.h

pasting from this file,

/*
 *      These inlines deal with timer wrapping correctly. You are
 *      strongly encouraged to use them
 *      1. Because people otherwise forget
 *      2. Because if the timer wrap changes in future you won't have to
 *         alter your driver code.
 *
 * time_after(a,b) returns true if the time a is after time b.
 *
 * Do this with "<0" and ">=0" to only test the sign of the result. A
 * good compiler would generate better code (and a really good compiler
 * wouldn't care). Gcc is currently neither.
 */
#define time_after(a,b)         \
        (typecheck(unsigned long, a) && \
         typecheck(unsigned long, b) && \
         ((long)(b) - (long)(a) < 0))
#define time_before(a,b)        time_after(b,a)
 
#define time_after_eq(a,b)      \
        (typecheck(unsigned long, a) && \
         typecheck(unsigned long, b) && \
         ((long)(a) - (long)(b) >= 0))
#define time_before_eq(a,b)     time_after_eq(b,a)
 



Derek.



On Mon, 22 Nov 2004, Peter Svensson wrote:

> On Mon, 22 Nov 2004, Gilad Ben-Yossef wrote:
> 
> > Dinesh Nair wrote:
> > >         origjiffies=jiffies;
> > >         while((jiffies-origjiffies)<4);
> 
> [snip]
> > One issue that is very important to consider is "jiffie warp". The 
> > jiffie is a 32 bit value, which means it will wrap around somewhere 
> > after a year for kernel 2.4/x86 and much sooner for 2.6 kernels or on 
> > the powerpc.
> > 
> > In the 2.6 series a jiffie_64 (or something similar, I don't remember) 
> > symbol exists, but on 32 bit arch's like the x86 it is not possible to 
> > read this value atomically without holding a lock. The regular jiffies 
> > symbols is really the 32 bit LSB of 64 bit jiffies value.
> > 
> > What all this means is the code snippet above is broken and will fail 
> > when the jiffies will wrap around causing untold damage to your machine, 
> > life and telephony equipment, but maybe not in that order... :-)
> 
> Actually it works nicely event at wrap around time, as long as the result 
> of the subtraction is a signed operation. Actually, I think there are 
> macros in the kernel that make sure the variables are cast correctly. 
> 
> Peter
> 
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