[Jack-Devel] How does --hwmon work?

[Chris Caudle]

I found this reply from Paul Davis (original jackd author/designer) for
essentially the same question from many years back:

---------------------------
On Fri, Aug 27, 2010 at 3:29 PM, Niels Mayer <[hidden email]> wrote:
> Finally reading the jackd manpage again (
> http://linux.die.net/man/1/jackd ) I notice
> -H, --hwmon

Most people do not use this. It is entirely replaced by using a h/w
specific utility (such as hdspmixer or envy2ctl) to control signal
routing. The one benefit that it has is that it means that JACK
*applications* can control that kind of routing, which early in the
life of Ardour seemed kind of important. It no longer seems very
important.
If you're already using a h/w specific utility for this purpose, then
don't use -H.

> PS: likewise what does the "-M, --hwmeter" option do -- again I don't
> see any difference between this being on or off. And how would it know

it doesn't really do anything. It was a placeholder for functionality
that has never been implemented because (a) so few cards implement it
and (b) the meter is basically useless for any app other than a h/w
specific utility app (which has other mechanisms available to get the
meter values).
---------------------------

So it looks like this feature never really became common, and it appears
that the jackd hardware monitor option only works on ICE or hdsp based
cards.  Maybe a few others, but unfortunately you would have to inspect
the ALSA driver source to make sure.

On Thu, May 4, 2017 4:17 am, David Kastrup wrote:
> I am currently in the process of Frankensteining a Roland FR1b "virtual
> accordion" with soft- and hardware Midi expanders, and the Roland only
> has a single Midi connector that is either in- or output.  So when the
> registration calls for the Roland to be silent, I have to access the
> mixer programmatically.

Which mixer?  The mixer in the audio interface?  I don't fully understand
the hardware configuration you are describing.

> Since I don't want the software to be tied to a particular soundcard, a
> more generic interface that _does_ use hardware monitoring/mixing of
> some kind internally when available would seem like the best fit.

In practical terms it seems you will be tied to a particular soundcard, or
at least family of cards, since it seems only the ICE/envy24 drivers and
the RME drivers implemented this feature.  Maybe a few others, but good
luck figuring out which.

> But it's not clear to me where which stuff is being done and whether,
> say, using jack_mix_box for the Roland sound control will magically buy
> me "zero-latency" monitoring or not.

Only if the ALSA driver reports has_hw_monitoring as true.
Which driver does the Roland sound control use?

> It really depends on what --hwmon does and what interfaces of Jack are
> involved.

>From a quick look at the jackd code it appears that the ALSA backend of
jackd queries the ALSA driver, and if the drivers reports
has_hw_monitoring then it sets the hw_monitoring property of the driver to
on if requested.  If the driver reports has_hw_monitoring as false then it
does nothing.

> the manual pages are vague and terribly outdated and

That is unfortunately true.  For most uses for which jack was designed, it
does what it is needed and has fallen into a barely maintained "just keep
it compiling" kind of state.  There is a new jackd maintainer, but I think
just working on it part time, and basic things like keeping man pages and
web pages up to date is not being done currently.

> Many ALSA cards have some sort of hardware mixer

Yes, but most of them have utilities for configuring the hardware mixer. 
The hwmon option to jackd is just on/off, it doesn't give any way to
control the gain of the connection, or the routing if there are multiple
inputs and multiple outputs.  I think the jackd option never caught on
because it was too limited to be useful.

-- 
Chris Caudle