It is no longer necessary to use jackd as a backend for Ardour in Linux. In fact with the spread of LV2 plugins, almost all workflows in Ardour work well with the ALSA backend. When using the ALSA backend for Ardour, Ardour will see all MIDI ports that ALSA sees without any user setup. However, should jackd need to be used, the rest of this page is valid.

The right approach for using MIDI on Linux depends on which version of JACK is in use. The world divides into:

Systems using JACK 1, versions 0.124 or later	On these systems, JACK must be started with the `-X alsa_midi` server argument. To support legacy control applications, the `-X seq` argument to the ALSA backend of JACK can also be used to get the exact same results.
All others	Using a2jmidid acts as a bridge between ALSA MIDI and JACK. The `-X seq` or `-X raw` arguments should not be used—the timing and performance of these options is unacceptable.

Using a2jmidid

a2jmidid is an application that bridges between the system MIDI ports and JACK.

First it must be ensured that there is no ALSA sequencer support enabled in JACK. To check that, one must open QJackCtl's Setup window and set Settings > MIDI Driver to none, then uncheck the Misc > Enable ALSA Sequencer support option. The jack server must then be restarted before going on.

Checking for a2jmidid availability

Next, it must be checked whether a2jmidid is already installed. This is done by starting the JACK server, then going to the command line and typing:

a2jmidid -e

If a2jmidid does not exist, it must be installed with the software manager of the Linux distribution in use until this command responds.

Checking available MIDI ports

If JACK is correctly configured for MIDI, then the MIDI ports should appear in qjackctl under Connections > MIDI.

Making it automatic

Once it has been verified that the ports appear in JACK as expected, this can be made to happen whenever JACK is started:

If a newer version of JACK 1 is in use, by just making sure the -X alsa_midi or -X seq options are enabled for whatever technique is being used to start JACK.
For other versions of JACK, by adding a2jmidid -e & as an "after start-up" script in the Setup > Options tab of QJackCtl, so that it is started automatically whenever JACK is started.