Re: Jiffy question and sound.

[watermodem <aquamodem@ameritech.net>]

Matti Aarnio wrote:
> 
> On Mon, Mar 19, 2001 at 12:20:47AM -0600, watermodem wrote:
> > With the 2.4.0 kernel the loops_per_sec field was replaced (for i386)
> > with current_cpu_data.loops_per_jiffy.
> ...
> > #define LOOPS_PER_SEC current_cpu_data.loops_per_jiffy * 100
> 
>   The intention was to accomodate systems with faster than 2 GHz clock
>   at which the LOOPS_PER_SEC counter spins around a bit too fast..
>   ('signed long' at i386 handles 0..2G just fine, then it thinks the sign
>    got inverted..  'unsigned long' works fine until 4 GHz processors.)
> 

My sound card uses ALSA and ALSA wasn't available yet for
the new kernel.  So.. Noting that LOOPS_PER_SEC was what 
failed in the kernel I modified it and compiled.  I am
not associated in anyway with the ALSA folks just wanted
too listen to music while working away.  I have no idea
why it needs it or if it is busy-looping... (I hope not).


>   Why does the ALSA need  LOOPS_PER_SEC ?
>   Is it doing timing by busy-looping ?
> 
> > Now compiling the same  ALSA modules with 2.4.2 this problem happens
> > much quicker and you don't need any other activity.  In fact it is hard
> > to play more than half a song.  (MP3)
> > It doesn't matter if what set of music players or tools I use the
> > problem is quite visible.
> >
> > When I boot back to the original 2.2.x kernel everything is perfect.
> >
> > So I guess I have a few questions here.
> >  1)   Is a jiffy 100th of a second or is it smaller  (so my loop count
> > is starving things.) (10ms) ?
> 
>         "HZ" is the answer.  E.g. Alpha has HZ=1024, while i386 has HZ=100
>         Nearly all architectures have different values based on what some
>         other UNIX uses at given system.
> 
> >  2)   Why is it so much worse in 2.4.2 than 2.4.0?
> >  3)   Any other "gotch's" that are important to watch for when moving
> > 2.2.x drivers to 2.4.x?
> 
>         The FAQ may have some pointers to "porting drivers to 2.4" documents.
> 
> > Thanks....
> > Watermodem
> > -
> > Please read the FAQ at  http://www.tux.org/lkml/
> 
> /Matti Aarnio