MIDI 2.0 on Linux


MIDI 2.0 is an extended protocol for providing higher resolutions and more fine controls over the legacy MIDI 1.0. The fundamental changes introduced for supporting MIDI 2.0 are:

  • Support of Universal MIDI Packet (UMP)

  • Support of MIDI 2.0 protocol messages

  • Transparent conversions between UMP and legacy MIDI 1.0 byte stream

  • MIDI-CI for property and profile configurations

UMP is a new container format to hold all MIDI protocol 1.0 and MIDI 2.0 protocol messages. Unlike the former byte stream, it's 32bit aligned, and each message can be put in a single packet. UMP can send the events up to 16 "UMP Groups", where each UMP Group contain up to 16 MIDI channels.

MIDI 2.0 protocol is an extended protocol to achieve the higher resolution and more controls over the old MIDI 1.0 protocol.

MIDI-CI is a high-level protocol that can talk with the MIDI device for the flexible profiles and configurations. It's represented in the form of special SysEx.

For Linux implementations, the kernel supports the UMP transport and the encoding/decoding of MIDI protocols on UMP, while MIDI-CI is supported in user-space over the standard SysEx.

As of this writing, only USB MIDI device supports the UMP and Linux 2.0 natively. The UMP support itself is pretty generic, hence it could be used by other transport layers, although it could be implemented differently (e.g. as a ALSA sequencer client), too.

The access to UMP devices are provided in two ways: the access via rawmidi device and the access via ALSA sequencer API.

ALSA sequencer API was extended to allow the payload of UMP packets. It's allowed to connect freely between MIDI 1.0 and MIDI 2.0 sequencer clients, and the events are converted transparently.

Kernel Configuration

The following new configs are added for supporting MIDI 2.0: CONFIG_SND_UMP, CONFIG_SND_UMP_LEGACY_RAWMIDI, CONFIG_SND_SEQ_UMP, CONFIG_SND_SEQ_UMP_CLIENT, and CONFIG_SND_USB_AUDIO_MIDI_V2. The first visible one is CONFIG_SND_USB_AUDIO_MIDI_V2, and when you choose it (to set =y), the core support for UMP (CONFIG_SND_UMP) and the sequencer binding (CONFIG_SND_SEQ_UMP_CLIENT) will be automatically selected.

Additionally, CONFIG_SND_UMP_LEGACY_RAWMIDI=y will enable the support for the legacy raw MIDI device for UMP Endpoints.

Rawmidi Device with USB MIDI 2.0

When a device supports MIDI 2.0, the USB-audio driver probes and uses the MIDI 2.0 interface (that is found always at the altset 1) as default instead of the MIDI 1.0 interface (at altset 0). You can switch back to the binding with the old MIDI 1.0 interface by passing midi2_enable=0 option to snd-usb-audio driver module, too.

The USB audio driver tries to query the UMP Endpoint and UMP Function Block information that are provided since UMP v1.1, and builds up the topology based on those information. When the device is older and doesn't respond to the new UMP inquiries, the driver falls back and builds the topology based on Group Terminal Block (GTB) information from the USB descriptor. Some device might be screwed up by the unexpected UMP command; in such a case, pass midi2_ump_probe=0 option to snd-usb-audio driver for skipping the UMP v1.1 inquiries.

When the MIDI 2.0 device is probed, the kernel creates a rawmidi device for each UMP Endpoint of the device. Its device name is /dev/snd/umpC*D* and different from the standard rawmidi device name /dev/snd/midiC*D* for MIDI 1.0, in order to avoid confusing the legacy applications accessing mistakenly to UMP devices.

You can read and write UMP packet data directly from/to this UMP rawmidi device. For example, reading via hexdump like below will show the incoming UMP packets of the card 0 device 0 in the hex format:

% hexdump -C /dev/snd/umpC0D0
00000000  01 07 b0 20 00 07 b0 20  64 3c 90 20 64 3c 80 20  |... ... d<. d<. |

Unlike the MIDI 1.0 byte stream, UMP is a 32bit packet, and the size for reading or writing the device is also aligned to 32bit (which is 4 bytes).

The 32-bit words in the UMP packet payload are always in CPU native endianness. Transport drivers are responsible to convert UMP words from / to system endianness to required transport endianness / byte order.

When CONFIG_SND_UMP_LEGACY_RAWMIDI is set, the driver creates another standard raw MIDI device additionally as /dev/snd/midiC*D*. This contains 16 substreams, and each substream corresponds to a (0-based) UMP Group. Legacy applications can access to the specified group via each substream in MIDI 1.0 byte stream format. With the ALSA rawmidi API, you can open the arbitrary substream, while just opening /dev/snd/midiC*D* will end up with opening the first substream.

Each UMP Endpoint can provide the additional information, constructed from the information inquired via UMP 1.1 Stream messages or USB MIDI 2.0 descriptors. And a UMP Endpoint may contain one or more UMP Blocks, where UMP Block is an abstraction introduced in the ALSA UMP implementations to represent the associations among UMP Groups. UMP Block corresponds to Function Block in UMP 1.1 specification. When UMP 1.1 Function Block information isn't available, it's filled partially from Group Terminal Block (GTB) as defined in USB MIDI 2.0 specifications.

The information of UMP Endpoints and UMP Blocks are found in the proc file /proc/asound/card*/midi*. For example:

% cat /proc/asound/card1/midi0

Type: UMP
EP Name: ProtoZOA
EP Product ID: ABCD12345678
UMP Version: 0x0000
Protocol Caps: 0x00000100
Protocol: 0x00000100
Num Blocks: 3

Block 0 (ProtoZOA Main)
  Direction: bidirection
  Active: Yes
  Groups: 1-1
  Is MIDI1: No

Block 1 (ProtoZOA Ext IN)
  Direction: output
  Active: Yes
  Groups: 2-2
  Is MIDI1: Yes (Low Speed)

Note that Groups field shown in the proc file above indicates the 1-based UMP Group numbers (from-to).

Those additional UMP Endpoint and UMP Block information can be obtained via the new ioctls SNDRV_UMP_IOCTL_ENDPOINT_INFO and SNDRV_UMP_IOCTL_BLOCK_INFO, respectively.

The rawmidi name and the UMP Endpoint name are usually identical, and in the case of USB MIDI, it's taken from iInterface of the corresponding USB MIDI interface descriptor. If it's not provided, it's copied from iProduct of the USB device descriptor as a fallback.

The Endpoint Product ID is a string field and supposed to be unique. It's copied from iSerialNumber of the device for USB MIDI.

The protocol capabilities and the actual protocol bits are defined in asound.h.

ALSA Sequencer with USB MIDI 2.0

In addition to the rawmidi interfaces, ALSA sequencer interface supports the new UMP MIDI 2.0 device, too. Now, each ALSA sequencer client may set its MIDI version (0, 1 or 2) to declare itself being either the legacy, UMP MIDI 1.0 or UMP MIDI 2.0 device, respectively. The first, legacy client is the one that sends/receives the old sequencer event as was. Meanwhile, UMP MIDI 1.0 and 2.0 clients send and receive in the extended event record for UMP. The MIDI version is seen in the new midi_version field of snd_seq_client_info.

A UMP packet can be sent/received in a sequencer event embedded by specifying the new event flag bit SNDRV_SEQ_EVENT_UMP. When this flag is set, the event has 16 byte (128 bit) data payload for holding the UMP packet. Without the SNDRV_SEQ_EVENT_UMP bit flag, the event is treated as a legacy event as it was (with max 12 byte data payload).

With SNDRV_SEQ_EVENT_UMP flag set, the type field of a UMP sequencer event is ignored (but it should be set to 0 as default).

The type of each client can be seen in /proc/asound/seq/clients. For example:

% cat /proc/asound/seq/clients
Client info
  cur  clients : 3
Client  14 : "Midi Through" [Kernel Legacy]
  Port   0 : "Midi Through Port-0" (RWe-)
Client  20 : "ProtoZOA" [Kernel UMP MIDI1]
  UMP Endpoint: ProtoZOA
  UMP Block 0: ProtoZOA Main [Active]
    Groups: 1-1
  UMP Block 1: ProtoZOA Ext IN [Active]
    Groups: 2-2
  UMP Block 2: ProtoZOA Ext OUT [Active]
    Groups: 3-3
  Port   0 : "MIDI 2.0" (RWeX) [In/Out]
  Port   1 : "ProtoZOA Main" (RWeX) [In/Out]
  Port   2 : "ProtoZOA Ext IN" (-We-) [Out]
  Port   3 : "ProtoZOA Ext OUT" (R-e-) [In]

Here you can find two types of kernel clients, "Legacy" for client 14, and "UMP MIDI1" for client 20, which is a USB MIDI 2.0 device. A USB MIDI 2.0 client gives always the port 0 as "MIDI 2.0" and the rest ports from 1 for each UMP Group (e.g. port 1 for Group 1). In this example, the device has three active groups (Main, Ext IN and Ext OUT), and those are exposed as sequencer ports from 1 to 3. The "MIDI 2.0" port is for a UMP Endpoint, and its difference from other UMP Group ports is that UMP Endpoint port sends the events from the all ports on the device ("catch-all"), while each UMP Group port sends only the events from the given UMP Group. Also, UMP groupless messages (such as the UMP message type 0x0f) are sent only to the UMP Endpoint port.

Note that, although each UMP sequencer client usually creates 16 ports, those ports that don't belong to any UMP Blocks (or belonging to inactive UMP Blocks) are marked as inactive, and they don't appear in the proc outputs. In the example above, the sequencer ports from 4 to 16 are present but not shown there.

The proc file above shows the UMP Block information, too. The same entry (but with more detailed information) is found in the rawmidi proc output.

When clients are connected between different MIDI versions, the events are translated automatically depending on the client's version, not only between the legacy and the UMP MIDI 1.0/2.0 types, but also between UMP MIDI 1.0 and 2.0 types, too. For example, running aseqdump program on the ProtoZOA Main port in the legacy mode will give you the output like:

% aseqdump -p 20:1
Waiting for data. Press Ctrl+C to end.
Source  Event                  Ch  Data
 20:1   Note on                 0, note 60, velocity 100
 20:1   Note off                0, note 60, velocity 100
 20:1   Control change          0, controller 11, value 4

When you run aseqdump in MIDI 2.0 mode, it'll receive the high precision data like:

% aseqdump -u 2 -p 20:1
Waiting for data. Press Ctrl+C to end.
Source  Event                  Ch  Data
 20:1   Note on                 0, note 60, velocity 0xc924, attr type = 0, data = 0x0
 20:1   Note off                0, note 60, velocity 0xc924, attr type = 0, data = 0x0
 20:1   Control change          0, controller 11, value 0x2000000

while the data is automatically converted by ALSA sequencer core.

Rawmidi API Extensions

  • The additional UMP Endpoint information can be obtained via the new ioctl SNDRV_UMP_IOCTL_ENDPOINT_INFO. It contains the associated card and device numbers, the bit flags, the protocols, the number of UMP Blocks, the name string of the endpoint, etc.

    The protocols are specified in two field, the protocol capabilities and the current protocol. Both contain the bit flags specifying the MIDI protocol version (SNDRV_UMP_EP_INFO_PROTO_MIDI1 or SNDRV_UMP_EP_INFO_PROTO_MIDI2) in the upper byte and the jitter reduction timestamp (SNDRV_UMP_EP_INFO_PROTO_JRTS_TX and SNDRV_UMP_EP_INFO_PROTO_JRTS_RX) in the lower byte.

    A UMP Endpoint may contain up to 32 UMP Blocks, and the number of the currently assigned blocks are shown in the Endpoint information.

  • Each UMP Block information can be obtained via another new ioctl SNDRV_UMP_IOCTL_BLOCK_INFO. The block ID number (0-based) has to be passed for the block to query. The received data contains the associated the direction of the block, the first associated group ID (0-based) and the number of groups, the name string of the block, etc.


  • For the device supports UMP v1.1, the UMP MIDI protocol can be switched via "Stream Configuration Request" message (UMP type 0x0f, status 0x05). When UMP core receives such a message, it updates the UMP EP info and the corresponding sequencer clients as well.

Control API Extensions

  • The new ioctl SNDRV_CTL_IOCTL_UMP_NEXT_DEVICE is introduced for querying the next UMP rawmidi device, while the existing ioctl SNDRV_CTL_IOCTL_RAWMIDI_NEXT_DEVICE queries only the legacy rawmidi devices.

    For setting the subdevice (substream number) to be opened, use the ioctl SNDRV_CTL_IOCTL_RAWMIDI_PREFER_SUBDEVICE like the normal rawmidi.

  • Two new ioctls SNDRV_CTL_IOCTL_UMP_ENDPOINT_INFO and SNDRV_CTL_IOCTL_UMP_BLOCK_INFO provide the UMP Endpoint and UMP Block information of the specified UMP device via ALSA control API without opening the actual (UMP) rawmidi device. The card field is ignored upon inquiry, always tied with the card of the control interface.

Sequencer API Extensions

  • midi_version field is added to snd_seq_client_info to indicate the current MIDI version (either 0, 1 or 2) of each client. When midi_version is 1 or 2, the alignment of read from a UMP sequencer client is also changed from the former 28 bytes to 32 bytes for the extended payload. The alignment size for the write isn't changed, but each event size may differ depending on the new bit flag below.

  • SNDRV_SEQ_EVENT_UMP flag bit is added for each sequencer event flags. When this bit flag is set, the sequencer event is extended to have a larger payload of 16 bytes instead of the legacy 12 bytes, and the event contains the UMP packet in the payload.

  • The new sequencer port type bit (SNDRV_SEQ_PORT_TYPE_MIDI_UMP) indicates the port being UMP-capable.

  • The sequencer ports have new capability bits to indicate the inactive ports (SNDRV_SEQ_PORT_CAP_INACTIVE) and the UMP Endpoint port (SNDRV_SEQ_PORT_CAP_UMP_ENDPOINT).

  • The event conversion of ALSA sequencer clients can be suppressed the new filter bit SNDRV_SEQ_FILTER_NO_CONVERT set to the client info. For example, the kernel pass-through client (snd-seq-dummy) sets this flag internally.

  • The port information gained the new field direction to indicate the direction of the port (either SNDRV_SEQ_PORT_DIR_INPUT, SNDRV_SEQ_PORT_DIR_OUTPUT or SNDRV_SEQ_PORT_DIR_BIDIRECTION).

  • Another additional field for the port information is ump_group which specifies the associated UMP Group Number (1-based). When it's non-zero, the UMP group field in the UMP packet updated upon delivery to the specified group (corrected to be 0-based). Each sequencer port is supposed to set this field if it's a port to specific to a certain UMP group.

  • Each client may set the additional event filter for UMP Groups in group_filter bitmap. The filter consists of bitmap from 1-based Group numbers. For example, when the bit 1 is set, messages from Group 1 (i.e. the very first group) are filtered and not delivered. The bit 0 is used for filtering UMP groupless messages.

  • Two new ioctls are added for UMP-capable clients: SNDRV_SEQ_IOCTL_GET_CLIENT_UMP_INFO and SNDRV_SEQ_IOCTL_SET_CLIENT_UMP_INFO. They are used to get and set either snd_ump_endpoint_info or snd_ump_block_info data associated with the sequencer client. The USB MIDI driver provides those information from the underlying UMP rawmidi, while a user-space client may provide its own data via *_SET ioctl. For an Endpoint data, pass 0 to the type field, while for a Block data, pass the block number + 1 to the type field. Setting the data for a kernel client shall result in an error.

  • With UMP 1.1, Function Block information may be changed dynamically. When the update of Function Block is received from the device, ALSA sequencer core changes the corresponding sequencer port name and attributes accordingly, and notifies the changes via the announcement to the ALSA sequencer system port, similarly like the normal port change notification.

MIDI2 USB Gadget Function Driver

The latest kernel contains the support for USB MIDI 2.0 gadget function driver, which can be used for prototyping and debugging MIDI 2.0 features.

CONFIG_USB_GADGET, CONFIG_USB_CONFIGFS and CONFIG_USB_CONFIGFS_F_MIDI2 need to be enabled for the MIDI2 gadget driver.

In addition, for using a gadget driver, you need a working UDC driver. In the example below, we use dummy_hcd driver (enabled via CONFIG_USB_DUMMY_HCD) that is available on PC and VM for debugging purpose. There are other UDC drivers depending on the platform, and those can be used for a real device, instead, too.

At first, on a system to run the gadget, load libcomposite module:

% modprobe libcomposite

and you'll have usb_gadget subdirectory under configfs space (typically /sys/kernel/config on modern OS). Then create a gadget instance and add configurations there, for example:

% cd /sys/kernel/config
% mkdir usb_gadget/g1

% cd usb_gadget/g1
% mkdir configs/c.1
% mkdir functions/midi2.usb0

% echo 0x0004 > idProduct
% echo 0x17b3 > idVendor
% mkdir strings/0x409
% echo "ACME Enterprises" > strings/0x409/manufacturer
% echo "ACMESynth" > strings/0x409/product
% echo "ABCD12345" > strings/0x409/serialnumber

% mkdir configs/c.1/strings/0x409
% echo "Monosynth" > configs/c.1/strings/0x409/configuration
% echo 120 > configs/c.1/MaxPower

At this point, there must be a subdirectory ep.0, and that is the configuration for a UMP Endpoint. You can fill the Endpoint information like:

% echo "ACMESynth" > functions/midi2.usb0/iface_name
% echo "ACMESynth" > functions/midi2.usb0/ep.0/ep_name
% echo "ABCD12345" > functions/midi2.usb0/ep.0/product_id
% echo 0x0123 > functions/midi2.usb0/ep.0/family
% echo 0x4567 > functions/midi2.usb0/ep.0/model
% echo 0x123456 > functions/midi2.usb0/ep.0/manufacturer
% echo 0x12345678 > functions/midi2.usb0/ep.0/sw_revision

The default MIDI protocol can be set either 1 or 2:

% echo 2 > functions/midi2.usb0/ep.0/protocol

And, you can find a subdirectory block.0 under this Endpoint subdirectory. This defines the Function Block information:

% echo "Monosynth" > functions/midi2.usb0/ep.0/block.0/name
% echo 0 > functions/midi2.usb0/ep.0/block.0/first_group
% echo 1 > functions/midi2.usb0/ep.0/block.0/num_groups

Finally, link the configuration and enable it:

% ln -s functions/midi2.usb0 configs/c.1
% echo dummy_udc.0 > UDC

where dummy_udc.0 is an example case and it differs depending on the system. You can find the UDC instances in /sys/class/udc and pass the found name instead:

% ls /sys/class/udc

Now, the MIDI 2.0 gadget device is enabled, and the gadget host creates a new sound card instance containing a UMP rawmidi device by f_midi2 driver:

% cat /proc/asound/cards
1 [Gadget         ]: f_midi2 - MIDI 2.0 Gadget
                     MIDI 2.0 Gadget

And on the connected host, a similar card should appear, too, but with the card and device names given in the configfs above:

% cat /proc/asound/cards
2 [ACMESynth      ]: USB-Audio - ACMESynth
                     ACME Enterprises ACMESynth at usb-dummy_hcd.0-1, high speed

You can play a MIDI file on the gadget side:

% aplaymidi -p 20:1 to_host.mid

and this will appear as an input from a MIDI device on the connected host:

% aseqdump -p 20:0 -u 2

Vice versa, a playback on the connected host will work as an input on the gadget, too.

Each Function Block may have different direction and UI-hint, specified via direction and ui_hint attributes. Passing 1 is for input-only, 2 for out-only and 3 for bidirectional (the default value). For example:

% echo 2 > functions/midi2.usb0/ep.0/block.0/direction
% echo 2 > functions/midi2.usb0/ep.0/block.0/ui_hint

When you need more than one Function Blocks, you can create subdirectories block.1, block.2, etc dynamically, and configure them in the configuration procedure above before linking. For example, to create a second Function Block for a keyboard:

% mkdir functions/midi2.usb0/ep.0/block.1
% echo "Keyboard" > functions/midi2.usb0/ep.0/block.1/name
% echo 1 > functions/midi2.usb0/ep.0/block.1/first_group
% echo 1 > functions/midi2.usb0/ep.0/block.1/num_groups
% echo 1 > functions/midi2.usb0/ep.0/block.1/direction
% echo 1 > functions/midi2.usb0/ep.0/block.1/ui_hint

The block.* subdirectories can be removed dynamically, too (except for block.0 which is persistent).

For assigning a Function Block for MIDI 1.0 I/O, set up in is_midi1 attribute. 1 is for MIDI 1.0, and 2 is for MIDI 1.0 with low speed connection:

% echo 2 > functions/midi2.usb0/ep.0/block.1/is_midi1

For disabling the processing of UMP Stream messages in the gadget driver, pass 0 to process_ump attribute in the top-level config:

% echo 0 > functions/midi2.usb0/process_ump

The MIDI 1.0 interface at altset 0 is supported by the gadget driver, too. When MIDI 1.0 interface is selected by the connected host, the UMP I/O on the gadget is translated from/to USB MIDI 1.0 packets accordingly while the gadget driver keeps communicating with the user-space over UMP rawmidi.

MIDI 1.0 ports are set up from the config in each Function Block. For example:

% echo 0 > functions/midi2.usb0/ep.0/block.0/midi1_first_group
% echo 1 > functions/midi2.usb0/ep.0/block.0/midi1_num_groups

The configuration above will enable the Group 1 (the index 0) for MIDI 1.0 interface. Note that those groups must be in the groups defined for the Function Block itself.

The gadget driver supports more than one UMP Endpoints, too. Similarly like the Function Blocks, you can create a new subdirectory ep.1 (but under the card top-level config) to enable a new Endpoint:

% mkdir functions/midi2.usb0/ep.1

and create a new Function Block there. For example, to create 4 Groups for the Function Block of this new Endpoint:

% mkdir functions/midi2.usb0/ep.1/block.0
% echo 4 > functions/midi2.usb0/ep.1/block.0/num_groups

Now, you'll have 4 rawmidi devices in total: the first two are UMP rawmidi devices for Endpoint 0 and Endpoint 1, and other two for the legacy MIDI 1.0 rawmidi devices corresponding to both EP 0 and EP 1.

The current altsetting on the gadget can be informed via a control element "Operation Mode" with RAWMIDI iface. e.g. you can read it via amixer program running on the gadget host like:

% amixer -c1 cget iface=RAWMIDI,name='Operation Mode'
; type=INTEGER,access=r--v----,values=1,min=0,max=2,step=0
: values=2

The value (shown in the second returned line with : values=) indicates 1 for MIDI 1.0 (altset 0), 2 for MIDI 2.0 (altset 1) and 0 for unset.

As of now, the configurations can't be changed after binding.