OSS Sequencer Emulation on ALSA


Copyright (c) 1998,1999 by Takashi Iwai <iwai@ww.uni-erlangen.de>

ver.0.1.5; May. 27, 1999


1. Description

This directory contains the OSS sequencer emulation driver on ALSA. Note that this program is still in the development state.

What this does - it provides the emulation of the OSS sequencer, access via /dev/sequencer device. /dev/music is not supported yet. The most of applications using OSS can run if the appropriate ALSA sequencer is prepared.

The following features are emulated by this driver:

Furthermore, you can find the following advanced features:

2. Installation

Run configure script with both sequencer support (--with-sequencer=yes) and OSS emulation (--with-oss=yes) options. A module snd-seq-oss.o will be created. If the synth module of your sound card supports for OSS emulation (for example, Emu8000 driver), this module will be loaded automatically. Otherwise, you need to load this module manually.

At beginning, this module probes all the MIDI ports which have been already connected to the sequencer. Once after that, the creation and deletion of ports are watched by announcement mechanism of ALSA sequencer.

The available synth and MIDI devices can be found in proc interface. Run "cat /proc/asound/seq/oss", and check the devices. For example, if you use an AWE64 card, you'll see like the following:

        OSS sequencer emulation version 0.1.4
        ALSA client number 128
        ALSA receiver port 0

        Number of applications: 0

        Number of synth devices: 1

        synth 0: EMU8000
          type 0x1
          subtype 0x20
          voices 32
          ioctl enabled
          load_patch enabled

        Number of MIDI devices: 3

        midi 0: Emu8000 Port-0
          client 21
          port 0
          capability write
          opened none

        midi 1: Emu8000 Port-1
          client 21
          port 1
          capability write
          opened none

        midi 2: 0: MPU-401 (UART)
          client 64
          port 0
          capability read/write
          opened none
Note that the device number may be different from the information of /proc/asound/oss-devices or ones of the original OSS driver. Use the device number listed in /proc/asound/seq/oss to play via OSS sequencer emulation.

3. Using Synthesizer Devices

Run your favorite program. I've tested playmidi-2.4, awemidi-0.4.3, gmod-3.1 and xmp-1.1.5. You can load samples via /dev/sequencer like sfxload, too.

If the lowlevel driver supports multiple access to synth devices (like Emu8000 driver), two or more applications are allowed to run at the same time.

4. Using MIDI Devices

So far, only MIDI output was tested. MIDI input was not checked at all, but hopefully it will work. Use the device number listed in /proc/asound/seq/oss. Be aware that these numbers are mostly different from the list in /proc/asound/oss-devices.

5. Module Options

The following module options are available:

6. Queue Mechanism

OSS sequencer emulation uses independent queues from ALSA sequencer. The events from /dev/sequencer are processed and put onto its own write-queue. Unlike the conventional OSS sequencer, the write-queue is managed as a time priority queue, as well as in ALSA sequencer.

All the events from /dev/sequencer are parsed at beginning. The timing events are also parsed at this moment, so that the events may be processed in real-time. Sending an event ABSTIME 0 switches the operation mode to real-time mode, and sending an event RELTIME 0 switches it off. In the real-time mode, all events are dispatched immediately.

The queued events are dispatched to the corresponding ALSA sequencer ports without enqueued on ALSA sequencer queue, or passed directly to the driver via callback. An ALSA timer is invoked to manage this schedule.

If the write-queue is full, the application sleeps until a certain amount (as default one half) becomes empty in blocking mode. The synchronization to write timing is implemented, too.

The input from MIDI devices or echo-back events are stored on read FIFO queue. If application reads /dev/sequencer in blocking mode, the process will be awaked.

                Synth Drivers
                load_patch, raw_event, etc.
                      ^ 
                      | (callbacks)                 ALSA sequencer
                      |
/dev/sequencer ---> write-queue -->(conversion)--> MIDI sequencer port
     ^                |                                 |
     |                | (echo)                          | (MIDI input)
     |                V                                 |
     + ------------ read-queue <---(conversion)<--------+

                  OSS emulation

7. Interface to Synthesizer Device

7.1. Registration

To register an OSS synthesizer device, use snd_seq_oss_synth_register function.
int snd_seq_oss_synth_register(char *name, int type, int subtype, int nvoices,
                              snd_seq_oss_callback_t *oper, void *private_data)
The arguments name, type, subtype and nvoices are used for making the appropriate synth_info structure for ioctl. The return value is an index number of this device. This index must be remembered for unregister. If registration is failed, -errno will be returned.

To release this device, call snd_seq_oss_synth_unregister function:

int snd_seq_oss_synth_unregister(int index),
where the index is the index number returned by register function.

7.2. Callbacks

OSS synthesizer devices have capability for sample downloading and ioctls like sample reset. In OSS emulation, these special features are realized by using callbacks. The registration argument oper is used to specify these callbacks. The following callback functions must be defined:
snd_seq_oss_callback_t:
        int (*open)(snd_seq_oss_arg_t *p, void *closure);
        int (*close)(snd_seq_oss_arg_t *p);
        int (*ioctl)(snd_seq_oss_arg_t *p, unsigned int cmd, unsigned long arg);
        int (*load_patch)(snd_seq_oss_arg_t *p, int format, const char *buf, int offs, int count);
        int (*reset)(snd_seq_oss_arg_t *p);
        int (*raw_event)(snd_seq_oss_arg_t *p, unsigned char *data);
Except for open and close callbacks, they are allowed to be NULL.

Each callback function takes the argument type snd_seq_oss_arg_t as the first argument.

struct snd_seq_oss_arg_t {
        int app_index;
        int file_mode;
        int seq_mode;
        snd_seq_addr_t addr;
        void *private_data;
        int event_passing;
};
The first three fields, app_index, file_mode and seq_mode are initialized by OSS sequencer. The app_index is the application index which is unique to each application opening OSS sequencer. The file_mode is bit-flags indicating the file operation mode. See seq_oss.h for its meaning. The seq_mode is sequencer operation mode. In the current version, only SND_OSSSEQ_MODE_SYNTH is used.

The next two fields, addr and private_data, must be filled by the synth driver at open callback. The addr contains the address of ALSA sequencer port which is assigned to this device. If the driver allocates memory for private_data, it must be released in close callback by itself.

The last field, event_passing, indicates how to translate note-on / off events. In PROCESS_EVENTS mode, the note 255 is regarded as velocity change, and key pressure event is passed to the port. In PASS_EVENTS mode, all note on/off events are passed to the port without modified. PROCESS_KEYPRESS mode checks the note above 128 and regards it as key pressure event (mainly for by Emu8000 driver).

7.2.1. Open Callback

The open is called at each time this device is opened by an application using OSS sequencer. This must not be NULL. Typically, the open callback does the following procedure:
  1. Allocate private data record.
  2. Create an ALSA sequencer port.
  3. Set the new port address on arg->addr.
  4. Set the private data record pointer on arg->private_data.
Note that the type bit-flags in port_info of this synth port must NOT contain TYPE_MIDI_GENERIC bit. Instead, TYPE_SPECIFIC should be used. Also, CAP_SUBSCRIPTION bit should NOT be included, too. This is necessary to tell it from other normal MIDI devices. If the open procedure succeeded, return zero. Otherwise, return -errno.

7.2.2 Ioctl Callback

The ioctl callback is called when the sequencer receives device-specific ioctls. The following two ioctls should be processed by this callback: The other ioctls are processed inside the sequencer without passing to the lowlevel driver.

7.2.3 Load_Patch Callback

The load_patch callback is used for sample-downloading. This callback must read the data on user-space and transfer to each device. Return 0 if succeeded, and -errno if failed. The format argument is the patch key in patch_info record. The buf is user-space pointer where patch_info record is stored. The offs can be ignored. The count is total data size of this sample data.

7.2.4 Close Callback

The close callback is called when this device is closed by the applicaion. If any private data was allocated in open callback, it must be released in the close callback. The deletion of ALSA port should be done here, too. This callback must not be NULL.

7.2.5 Reset Callback

The reset callback is called when sequencer device is reset or closed by applications. The callback should turn off the sounds on the relevant port immediately, and initialize the status of the port. If this callback is undefined, OSS seq sends a HEARTBEAT event to the port.

7.2.6 Raw_Event Callback

The raw_event callback is called when SEQ_PRIVATE or SEQ_VOLUME events are processed. The OSS sequencer passes these event 8 byte packets without any modification. The lowlevel driver should process these events appropriately.

7.3 Normal Events

Most of the events are processed by sequencer and translated to the adequate ALSA sequencer events, so that each synth device can receive by input_event callback of ALSA sequencer port. The following ALSA events should be implemented by the driver:
 
ALSA event Original OSS events
NOTEON SEQ_NOTEON
MIDI_NOTEON
NOTE SEQ_NOTEOFF
MIDI_NOTEOFF
KEYPRESS MIDI_KEY_PRESSURE
CHANPRESS SEQ_AFTERTOUCH
MIDI_CHN_PRESSURE
PGMCHANGE SEQ_PGMCHANGE
MIDI_PGM_CHANGE
PITCHBEND SEQ_CONTROLLER(CTRL_PITCH_BENDER)
MIDI_PITCH_BEND
CONTROLLER MIDI_CTL_CHANGE
SEQ_BALANCE (with CTL_PAN)
CONTROL14 SEQ_CONTROLLER
REGPARAM SEQ_CONTROLLER(CTRL_PITCH_BENDER_RANGE)
SYSEX SEQ_SYSEX

The most of these behavior can be realized by MIDI emulation driver included in the Emu8000 lowlevel driver. In the future release, this module will be independent.

8. Interface to MIDI Device

Since the OSS emulation probes the creation and deletion of ALSA MIDI sequencer ports automatically by receiving announcement from ALSA sequencer, the MIDI devices don't need to be registered explicitly like synth devices. However, the MIDI port_info registered to ALSA sequencer must include a type-bit MIDI_GENERIC and a capability-bit CAP_SUBSCRIPTION. Also, either CAP_IN or CAP_OUT must be defined. If these conditions are not satisfied, the port is not registered as OSS sequencer MIDI device.

The events via MIDI devices are parsed in OSS sequencer and converted to the corresponding ALSA sequencer events. The input from MIDI sequencer is also converted to MIDI byte events by OSS sequencer. This works just a reverse way of seq_midi module.

9. Known Problems