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                       OSS Sequencer Emulation on ALSA

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

ver.0.1.8; Nov. 16, 1999

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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 and /dev/music devices. The most of applications using OSS
can run if the appropriate ALSA sequencer is prepared.

The following features are emulated by this driver:

   * Normal sequencer and MIDI events:
     They are converted to the ALSA sequencer events, and sent to the
     corresponding port.
   * Timer events:
     The timer is not selectable by ioctl. The control rate is fixed to 100
     regardless of HZ. That is, even on Alpha system, a tick is always 1/100
     second. The base rate and tempo can be changed in /dev/music.
   * Patch loading:
     It purely depends on the synth drivers whether it's supported since the
     patch loading is realized by callback to the synth driver.
   * I/O controls:
     Most of controls are accepted. Some controls are dependent on the
     synth driver, as well as even on original OSS.

Furthermore, you can find the following advanced features:

   * Better queue mechanism:
     The events are queued before processing them.
   * Multiple applications:
     You can run two or more applications simultaneously (even for OSS
     sequencer)! However, each MIDI device is exclusive - that is, if a MIDI
     device is opened once by some application, other applications can't use
     it. No such a restriction in synth devices.
   * Real-time event processing:
     The events can be processed in real time without using out of bound
     ioctl. To switch to real-time mode, send ABSTIME 0 event. The followed
     events will be processed in real-time without queued. To switch off the
     real-time mode, send RELTIME 0 event.
   * /proc interface:
     The status of applications and devices can be shown via
     /proc/asound/seq/oss at any time. In the later version, configuration
     will be changed via /proc interface, too.

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
(so far, only 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 0.1.8
        ALSA client number 63
        ALSA receiver port 0

        Number of applications: 0

        Number of synth devices: 1

        synth 0: [EMU8000]
          type 0x1 : subtype 0x20 : voices 32
          capabilities : ioctl enabled / load_patch enabled

        Number of MIDI devices: 3

        midi 0: [Emu8000 Port-0] ALSA port 65:0
          capability write / opened none

        midi 1: [Emu8000 Port-1] ALSA port 65:1
          capability write / opened none

        midi 2: [0: MPU-401 (UART)] ALSA port 64:0
          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:

   * maxqlen
     specifies the maximum read/write queue length. This queue is private
     for OSS sequencer, so that it is independent from the queue length of
     ALSA sequencer. Default value is 1024.
   * seq_oss_debug
     specifies the debug level and accepts zero (= no debug message) or
     positive integer. Default value is 0.

6. Queue Mechanism

OSS sequencer emulation uses an ALSA priority queue. The events from
/dev/sequencer are processed and put onto the queue.

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
after scheduled time by ALSA sequencer dispatcher.

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 was 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.

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);

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
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:

   * IOCTL_SEQ_RESET_SAMPLES
     reset all samples on memory -- return 0
   * IOCTL_SYNTH_MEMAVL
     return the available memory size
   * FM_4OP_ENABLE
     can be ignored usually

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 application. 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.

7.3 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.

Some OSS events (SEQ_PRIVATE and SEQ_VOLUME events) are passed as event
type SND_SEQ_OSS_PRIVATE.  The OSS sequencer passes these event 8 byte
packets without any modification. The lowlevel driver should process these
events appropriately.

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 group
name SND_SEQ_GROUP_DEVICE and a capability-bit CAP_READ or CAP_WRITE. Also,
subscription capabilities, CAP_SUBS_READ or CAP_SUBS_WRITE, must be defined,
too. 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 / TODO's

   * Patch loading via ALSA instrument layer is not implemented yet.

