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

[Gilad Ben-Yossef]

Dinesh Nair wrote:
> 
> hi all,
> 
> i'm trying to reconcile the register settings and proslic calibration 
> procedure for the wcfxs/wctdm driver between the linux zaptel drivers 
> and the freebsd zaptel drivers. i'm pretty much sure that a lot of the 
> problems is timing (i.e. wait/sleep time) based differences and a lot of 
> this is due to the assumption of what the variable jiffie contains.
> 
> i do believe that it is the number of clock ticks, however what's the 
> frequency of the clock ticks ? is it 100 per second or 1000 per second ? 
> this has a vast difference when used in code like this:
> 
>         origjiffies=jiffies;
>         while((jiffies-origjiffies)<4);
> 
> is the above while looping for 4ms or 40ms ? the lack of comments in the 
> file make it difficult to port, for freebsd's clock ticks are defined by 
> the HZ kernel variable, which by default is set to 100, but can be 
> increased (and is usually increased) to 1000 for performance sensitive 
> deployments.
> 
> can anyone assist in clarifying what a jiffie is ?
> 

Under Linux the jiffies variable is the number of clock ticks since the 
machine boot. It's rate is defined by (surprise, surprise...) the HZ 
define from the kernel param.h

For a normal (as in - not patched) Linux kernel of versions 2.4.x the 
value of HZ is 100 for the x86 platform and a 1000 for the PowerPC 
platform. Since version 2.6 the x86 port also has a value of 1000 for HZ 
  by default. Patches exist to make these variable during compile time.

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... :-)

Hope this helped, or at least confused you in a pleasurable fashion.

Cheers,
Gilad

-- 
Gilad Ben-Yossef <gilad at codefidence.com>
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