ALSA: docs: Add MIDI 2.0 documentation

Add the brief document for describing the MIDI 2.0 implementation on
Linux kernel.  Both rawmidi and sequencer API extensions are
described.

Acked-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230523075358.9672-38-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: tiwai

Documentation/sound/designs/index.rst

@@ -15,3 +15,4 @@ Designs and Implementations
    oss-emulation
    seq-oss
    jack-injection
+   midi-2.0

Documentation/sound/designs/midi-2.0.rst

@@ -0,0 +1,351 @@
+=================
+MIDI 2.0 on Linux
+=================
+
+General
+=======
+
+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.
+
+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 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 Group Terminal Block (GTB) in USB
+MIDI 2.0 specifications but provide a few more generic information.
+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
+  ProtoZOA MIDI
+  
+  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.
+
+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.
+
+  The direction is either `SNDRV_UMP_DIR_INPUT`,
+  `SNDRV_UMP_DIR_OUTPUT` or `SNDRV_UMP_DIR_BIDIRECTION`.
+
+
+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 reserved for future use.
+
+* 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.