372 lines
13 KiB
ReStructuredText
372 lines
13 KiB
ReStructuredText
===============================
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OSS Sequencer Emulation on ALSA
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===============================
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Copyright (c) 1998,1999 by Takashi Iwai
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ver.0.1.8; Nov. 16, 1999
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Description
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===========
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This directory contains the OSS sequencer emulation driver on ALSA. Note
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that this program is still in the development state.
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What this does - it provides the emulation of the OSS sequencer, access
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via ``/dev/sequencer`` and ``/dev/music`` devices.
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The most of applications using OSS can run if the appropriate ALSA
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sequencer is prepared.
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The following features are emulated by this driver:
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* Normal sequencer and MIDI events:
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They are converted to the ALSA sequencer events, and sent to the
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corresponding port.
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* Timer events:
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The timer is not selectable by ioctl. The control rate is fixed to
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100 regardless of HZ. That is, even on Alpha system, a tick is always
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1/100 second. The base rate and tempo can be changed in ``/dev/music``.
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* Patch loading:
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It purely depends on the synth drivers whether it's supported since
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the patch loading is realized by callback to the synth driver.
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* I/O controls:
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Most of controls are accepted. Some controls
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are dependent on the synth driver, as well as even on original OSS.
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Furthermore, you can find the following advanced features:
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* Better queue mechanism:
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The events are queued before processing them.
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* Multiple applications:
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You can run two or more applications simultaneously (even for OSS
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sequencer)!
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However, each MIDI device is exclusive - that is, if a MIDI device
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is opened once by some application, other applications can't use
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it. No such a restriction in synth devices.
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* Real-time event processing:
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The events can be processed in real time without using out of bound
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ioctl. To switch to real-time mode, send ABSTIME 0 event. The followed
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events will be processed in real-time without queued. To switch off the
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real-time mode, send RELTIME 0 event.
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* ``/proc`` interface:
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The status of applications and devices can be shown via
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``/proc/asound/seq/oss`` at any time. In the later version,
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configuration will be changed via ``/proc`` interface, too.
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Installation
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============
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Run configure script with both sequencer support (``--with-sequencer=yes``)
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and OSS emulation (``--with-oss=yes``) options. A module ``snd-seq-oss.o``
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will be created. If the synth module of your sound card supports for OSS
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emulation (so far, only Emu8000 driver), this module will be loaded
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automatically.
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Otherwise, you need to load this module manually.
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At beginning, this module probes all the MIDI ports which have been
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already connected to the sequencer. Once after that, the creation and deletion
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of ports are watched by announcement mechanism of ALSA sequencer.
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The available synth and MIDI devices can be found in proc interface.
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Run ``cat /proc/asound/seq/oss``, and check the devices. For example,
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if you use an AWE64 card, you'll see like the following:
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::
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OSS sequencer emulation version 0.1.8
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ALSA client number 63
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ALSA receiver port 0
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Number of applications: 0
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Number of synth devices: 1
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synth 0: [EMU8000]
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type 0x1 : subtype 0x20 : voices 32
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capabilities : ioctl enabled / load_patch enabled
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Number of MIDI devices: 3
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midi 0: [Emu8000 Port-0] ALSA port 65:0
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capability write / opened none
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midi 1: [Emu8000 Port-1] ALSA port 65:1
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capability write / opened none
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midi 2: [0: MPU-401 (UART)] ALSA port 64:0
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capability read/write / opened none
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Note that the device number may be different from the information of
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``/proc/asound/oss-devices`` or ones of the original OSS driver.
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Use the device number listed in ``/proc/asound/seq/oss``
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to play via OSS sequencer emulation.
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Using Synthesizer Devices
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=========================
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Run your favorite program. I've tested playmidi-2.4, awemidi-0.4.3, gmod-3.1
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and xmp-1.1.5. You can load samples via ``/dev/sequencer`` like sfxload,
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too.
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If the lowlevel driver supports multiple access to synth devices (like
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Emu8000 driver), two or more applications are allowed to run at the same
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time.
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Using MIDI Devices
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==================
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So far, only MIDI output was tested. MIDI input was not checked at all,
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but hopefully it will work. Use the device number listed in
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``/proc/asound/seq/oss``.
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Be aware that these numbers are mostly different from the list in
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``/proc/asound/oss-devices``.
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Module Options
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==============
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The following module options are available:
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maxqlen
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specifies the maximum read/write queue length. This queue is private
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for OSS sequencer, so that it is independent from the queue length of ALSA
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sequencer. Default value is 1024.
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seq_oss_debug
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specifies the debug level and accepts zero (= no debug message) or
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positive integer. Default value is 0.
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Queue Mechanism
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===============
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OSS sequencer emulation uses an ALSA priority queue. The
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events from ``/dev/sequencer`` are processed and put onto the queue
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specified by module option.
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All the events from ``/dev/sequencer`` are parsed at beginning.
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The timing events are also parsed at this moment, so that the events may
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be processed in real-time. Sending an event ABSTIME 0 switches the operation
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mode to real-time mode, and sending an event RELTIME 0 switches it off.
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In the real-time mode, all events are dispatched immediately.
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The queued events are dispatched to the corresponding ALSA sequencer
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ports after scheduled time by ALSA sequencer dispatcher.
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If the write-queue is full, the application sleeps until a certain amount
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(as default one half) becomes empty in blocking mode. The synchronization
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to write timing was implemented, too.
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The input from MIDI devices or echo-back events are stored on read FIFO
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queue. If application reads ``/dev/sequencer`` in blocking mode, the
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process will be awaked.
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Interface to Synthesizer Device
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===============================
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Registration
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------------
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To register an OSS synthesizer device, use snd_seq_oss_synth_register()
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function:
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::
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int snd_seq_oss_synth_register(char *name, int type, int subtype, int nvoices,
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snd_seq_oss_callback_t *oper, void *private_data)
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The arguments ``name``, ``type``, ``subtype`` and ``nvoices``
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are used for making the appropriate synth_info structure for ioctl. The
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return value is an index number of this device. This index must be remembered
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for unregister. If registration is failed, -errno will be returned.
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To release this device, call snd_seq_oss_synth_unregister() function:
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::
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int snd_seq_oss_synth_unregister(int index)
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where the ``index`` is the index number returned by register function.
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Callbacks
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---------
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OSS synthesizer devices have capability for sample downloading and ioctls
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like sample reset. In OSS emulation, these special features are realized
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by using callbacks. The registration argument oper is used to specify these
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callbacks. The following callback functions must be defined:
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::
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snd_seq_oss_callback_t:
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int (*open)(snd_seq_oss_arg_t *p, void *closure);
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int (*close)(snd_seq_oss_arg_t *p);
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int (*ioctl)(snd_seq_oss_arg_t *p, unsigned int cmd, unsigned long arg);
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int (*load_patch)(snd_seq_oss_arg_t *p, int format, const char *buf, int offs, int count);
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int (*reset)(snd_seq_oss_arg_t *p);
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Except for ``open`` and ``close`` callbacks, they are allowed to be NULL.
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Each callback function takes the argument type ``snd_seq_oss_arg_t`` as the
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first argument.
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::
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struct snd_seq_oss_arg_t {
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int app_index;
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int file_mode;
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int seq_mode;
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snd_seq_addr_t addr;
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void *private_data;
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int event_passing;
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};
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The first three fields, ``app_index``, ``file_mode`` and ``seq_mode``
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are initialized by OSS sequencer. The ``app_index`` is the application
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index which is unique to each application opening OSS sequencer. The
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``file_mode`` is bit-flags indicating the file operation mode. See
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``seq_oss.h`` for its meaning. The ``seq_mode`` is sequencer operation
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mode. In the current version, only ``SND_OSSSEQ_MODE_SYNTH`` is used.
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The next two fields, ``addr`` and ``private_data``, must be
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filled by the synth driver at open callback. The ``addr`` contains
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the address of ALSA sequencer port which is assigned to this device. If
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the driver allocates memory for ``private_data``, it must be released
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in close callback by itself.
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The last field, ``event_passing``, indicates how to translate note-on
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/ off events. In ``PROCESS_EVENTS`` mode, the note 255 is regarded
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as velocity change, and key pressure event is passed to the port. In
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``PASS_EVENTS`` mode, all note on/off events are passed to the port
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without modified. ``PROCESS_KEYPRESS`` mode checks the note above 128
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and regards it as key pressure event (mainly for Emu8000 driver).
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Open Callback
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-------------
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The ``open`` is called at each time this device is opened by an application
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using OSS sequencer. This must not be NULL. Typically, the open callback
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does the following procedure:
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#. Allocate private data record.
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#. Create an ALSA sequencer port.
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#. Set the new port address on ``arg->addr``.
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#. Set the private data record pointer on ``arg->private_data``.
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Note that the type bit-flags in port_info of this synth port must NOT contain
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``TYPE_MIDI_GENERIC``
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bit. Instead, ``TYPE_SPECIFIC`` should be used. Also, ``CAP_SUBSCRIPTION``
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bit should NOT be included, too. This is necessary to tell it from other
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normal MIDI devices. If the open procedure succeeded, return zero. Otherwise,
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return -errno.
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Ioctl Callback
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--------------
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The ``ioctl`` callback is called when the sequencer receives device-specific
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ioctls. The following two ioctls should be processed by this callback:
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IOCTL_SEQ_RESET_SAMPLES
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reset all samples on memory -- return 0
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IOCTL_SYNTH_MEMAVL
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return the available memory size
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FM_4OP_ENABLE
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can be ignored usually
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The other ioctls are processed inside the sequencer without passing to
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the lowlevel driver.
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Load_Patch Callback
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-------------------
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The ``load_patch`` callback is used for sample-downloading. This callback
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must read the data on user-space and transfer to each device. Return 0
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if succeeded, and -errno if failed. The format argument is the patch key
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in patch_info record. The buf is user-space pointer where patch_info record
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is stored. The offs can be ignored. The count is total data size of this
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sample data.
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Close Callback
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--------------
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The ``close`` callback is called when this device is closed by the
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application. If any private data was allocated in open callback, it must
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be released in the close callback. The deletion of ALSA port should be
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done here, too. This callback must not be NULL.
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Reset Callback
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--------------
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The ``reset`` callback is called when sequencer device is reset or
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closed by applications. The callback should turn off the sounds on the
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relevant port immediately, and initialize the status of the port. If this
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callback is undefined, OSS seq sends a ``HEARTBEAT`` event to the
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port.
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Events
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======
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Most of the events are processed by sequencer and translated to the adequate
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ALSA sequencer events, so that each synth device can receive by input_event
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callback of ALSA sequencer port. The following ALSA events should be
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implemented by the driver:
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============= ===================
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ALSA event Original OSS events
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============= ===================
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NOTEON SEQ_NOTEON, MIDI_NOTEON
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NOTE SEQ_NOTEOFF, MIDI_NOTEOFF
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KEYPRESS MIDI_KEY_PRESSURE
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CHANPRESS SEQ_AFTERTOUCH, MIDI_CHN_PRESSURE
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PGMCHANGE SEQ_PGMCHANGE, MIDI_PGM_CHANGE
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PITCHBEND SEQ_CONTROLLER(CTRL_PITCH_BENDER),
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MIDI_PITCH_BEND
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CONTROLLER MIDI_CTL_CHANGE,
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SEQ_BALANCE (with CTL_PAN)
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CONTROL14 SEQ_CONTROLLER
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REGPARAM SEQ_CONTROLLER(CTRL_PITCH_BENDER_RANGE)
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SYSEX SEQ_SYSEX
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============= ===================
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The most of these behavior can be realized by MIDI emulation driver
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included in the Emu8000 lowlevel driver. In the future release, this module
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will be independent.
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Some OSS events (``SEQ_PRIVATE`` and ``SEQ_VOLUME`` events) are passed as event
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type SND_SEQ_OSS_PRIVATE. The OSS sequencer passes these event 8 byte
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packets without any modification. The lowlevel driver should process these
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events appropriately.
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Interface to MIDI Device
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========================
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Since the OSS emulation probes the creation and deletion of ALSA MIDI
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sequencer ports automatically by receiving announcement from ALSA
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sequencer, the MIDI devices don't need to be registered explicitly
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like synth devices.
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However, the MIDI port_info registered to ALSA sequencer must include
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a group name ``SND_SEQ_GROUP_DEVICE`` and a capability-bit
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``CAP_READ`` or ``CAP_WRITE``. Also, subscription capabilities,
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``CAP_SUBS_READ`` or ``CAP_SUBS_WRITE``, must be defined, too. If
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these conditions are not satisfied, the port is not registered as OSS
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sequencer MIDI device.
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The events via MIDI devices are parsed in OSS sequencer and converted
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to the corresponding ALSA sequencer events. The input from MIDI sequencer
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is also converted to MIDI byte events by OSS sequencer. This works just
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a reverse way of seq_midi module.
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Known Problems / TODO's
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=======================
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* Patch loading via ALSA instrument layer is not implemented yet.
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