RTC

** Note **
this text is quite outdated.  If you check out the linux kernels
high resolution timer patches for 2.4 (they are built in 2.5+).
They will allow the scheduler and default timers to run at 1000Hz.

[1] RTC - What is is....

The following was taken from the site of Takashi Iwai:
(http://www.alsa-project.org/~iwai/alsa.html)
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RTC High Frequency Timer Patch

RTC has a nice high-frequent interrupt capability up to 8192 Hz. Although 
this can be used (on the recent kernel) even from userspace via fasync 
signals, it is not allowed to use the interrupt from kernel. This is the 
very reason of this patch.

This tiny patch adds a hook to the RTC driver in linux kernel. A callback 
function is called at each interrupt. For changing frequency, starting and 
stopping interrupts, use rtc_control function.

The older version of this patch had more functionalities than the current 
version, for example, multiple callbacks with different frequencies are 
allowed. From this patch, however, I intensionally removed these things, 
because of simplicity. Only one callback can be used exclusively (even 
to user-space). That is, if a kernel driver (e.g. ALSA RTC timer) registers 
the callback, no other threads can use RTC interrupt. /dev/rtc cannot be 
opened during it.
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[2] What is does.....

In a nutshell, it allows the sequencer to pump out midi messages at a 
resolution of 1 millisecond, compared to the standard 2.4.x kernels 10ms.  
This is needed for midi clock, where 2-3 ms in-between ticks is necessary,
or your synced gear will be acting quite funny.

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[3] How to get it...

So basically, if you want to use the RTC patch, you have to get it from 
the alsa-driver files.  

Download the current driver tarball from ( http://www.alsa-project.org/ ).  

Located in [alsa-driver-0.9.0x/utils/patches] are diff files to apply 
towards your kernel source.

Once you have that patched and installed, you compile your alsa drivers 
with the rtc-timer module.

Then, modify your modules.conf file accordingly.  
Here is what mine looks like:


> modules.conf excerpt ------------------

# rtc
options snd-timer snd_timer_limit=2
alias snd-timer-1 snd-rtctimer
options snd-seq snd_seq_default_timer_resolution=1000 
options snd-seq snd_seq_default_timer_device=1

# ALSA portion
alias char-major-116 snd
alias snd-card-0 snd-emu10k1

> --------------------------------------

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[4] Problems ?

If your having problems with the RTC module, please ask on one of the 
ALSA lists.  The author of seq24 does not wish to troubleshoot your 
kernel/alsa issues.