Using hardware timers for the ALSA sequencer

Using hardware timers for the ALSA sequencer

[Lee Revell]

Hey,

The emu10k1 timer thread reminded me of an idea I had.

Currently the ALSA sequencer always uses the system timer by default.
If your sound cards timer is supported by ALSA (emu10k1 and ymfpci),
this provides 50-100x better resolution.  But in order to get the ALSA
sequencer to use it you have to use the seq_default_timer_*  options
(which BTW I had never heard of on the list, I stumbled across them
reading the source).

How hard would it be to get the sequencer to prefer the hardware timer
by default and fall back to the system timer otherwise?  Probably would
not be noticeable for a MIDI keyboard, but for applications like sending
MIDI clock/MTC, the system timer resolution can be inadequate.

Would this be a good idea?

Lee





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Re: Using hardware timers for the ALSA sequencer

[Clemens Ladisch]

Lee Revell wrote:
> How hard would it be to get the sequencer to prefer the hardware timer
> by default and fall back to the system timer otherwise?

Probably not very hard -- registering a hardware timer would change
the default timer, or something like that.

> Probably would not be noticeable for a MIDI keyboard, but for
> applications like sending MIDI clock/MTC, the system timer
> resolution can be inadequate.

For USB MIDI ports, 1000 Hz is adequate, and that is the system timer
resolution on 2.6 kernels.

The theoretical upper limit for other ports would be the MIDI bit
clock, i.e., 31250 Hz. :)

> Would this be a good idea?

I think the only application where a higher resolution would be useful
would be sending MIDI clock/MTC, and we would want a kernel module for
this to prevent _any_ scheduling latencies.  A special module like
this could easily have its own mechanism for selecting the timer to
use.


Regards,
Clemens



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Re: Using hardware timers for the ALSA sequencer

[Lee Revell]

On Tue, 2005-03-15 at 09:55 +0100, Clemens Ladisch wrote:
> For USB MIDI ports, 1000 Hz is adequate, and that is the system timer
> resolution on 2.6 kernels.

So how does MIDI work with 2.4?  Not very well?

Lee



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Re: Using hardware timers for the ALSA sequencer

[Clemens Ladisch]

Lee Revell wrote:
> On Tue, 2005-03-15 at 09:55 +0100, Clemens Ladisch wrote:
> > For USB MIDI ports, 1000 Hz is adequate, and that is the system timer
> > resolution on 2.6 kernels.
>
> So how does MIDI work with 2.4?  Not very well?

With the default settings, MIDI playback will have jitter up to 10 ms.
A few people use the RTC timer or a sound card timer instead.


Regards,
Clemens



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Re: Using hardware timers for the ALSA sequencer

[Lee Revell]

On Tue, 2005-03-15 at 09:55 +0100, Clemens Ladisch wrote:
> Probably not very hard -- registering a hardware timer would change
> the default timer, or something like that.

I was thinking even simpler.  If a hardware timer is present when the
sequencer is initialized, use it.  Handling the registration of a
hardware timer once the sequencer is up seems harder.

Lee



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Re: Using hardware timers for the ALSA sequencer

[Manuel Jander]

Hi,

At least the aureal vortex has a high resolution hardware timer too. So
if this leads to some code to support hardware timers, i would suggest
something useable for other hardware too.

Best Regards

Manuel J. 

On Mon, 2005-03-14 at 19:07 -0500, Lee Revell wrote:
> Hey,
> 
> The emu10k1 timer thread reminded me of an idea I had.
> 
> Currently the ALSA sequencer always uses the system timer by default.
> If your sound cards timer is supported by ALSA (emu10k1 and ymfpci),
> this provides 50-100x better resolution.  But in order to get the ALSA
> sequencer to use it you have to use the seq_default_timer_*  options
> (which BTW I had never heard of on the list, I stumbled across them
> reading the source).
> 
> How hard would it be to get the sequencer to prefer the hardware timer
> by default and fall back to the system timer otherwise?  Probably would
> not be noticeable for a MIDI keyboard, but for applications like sending
> MIDI clock/MTC, the system timer resolution can be inadequate.
> 
> Would this be a good idea?
> 
> Lee
> 
> 
> 
> 
> 
> -------------------------------------------------------
> SF email is sponsored by - The IT Product Guide
> Read honest & candid reviews on hundreds of IT Products from real users.
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> _______________________________________________
> Alsa-devel mailing list
> Alsa-devel@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/alsa-devel
-- 
Manuel Jander
Electronic Engineer



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Re: Using hardware timers for the ALSA sequencer

[Lee Revell]

On Wed, 2005-03-16 at 20:21 -0400, Manuel Jander wrote:
> Hi,
> 
> At least the aureal vortex has a high resolution hardware timer too. So
> if this leads to some code to support hardware timers, i would suggest
> something useable for other hardware too.
> 

With Clemens' code for the ymfpci timer and the OSS emu10k1 driver I was
able to implement the ALSA timer support for emu10k1 in about 20
minutes.  If you know how to enable and disable timer interrupts and set
the timer rate (and have the hardware of course) you should try to
implement the support for the vortex.

Lee



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