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:
-
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, but only by module option. The
base rate and tempo are also fixed. The control rate is fixed to 100 regardless
of HZ. That is, even on Alpha system, a tick is always 1/100 second. This
will be configurable in the later version.
-
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. Timer controls are ignored. 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 (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:
-
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.
-
oss_queue
specifies the queue number of ALSA sequencer. The queue will be used
only to receive announcements of port creation and deletion. Default value
is 7.
-
timer
specifies the timer number to be used. As default, the system timer
(number 0) is used. More precise timer can be chosen by this option. To
know what timers can be used, see /proc/asound/timer. Default
value is 0.
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:
-
Allocate private data record.
-
Create an ALSA sequencer port.
-
Set the new port address on arg->addr.
-
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
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
-
Compiling by egcs-1.1.1 and 1.1.2-pre3 may cause kernel Oops. gcc-2.7.2.x
works fine.
(Using egcs-1.1.2 seems good, so far -- iwai)
-
The snd-seq-oss.o is listed as dependent module of some other OSS
modules like awe_wave.o (at least on my machine). I have no idea
why this happens... This problem is harmless to ALSA, but OSS module can't
be loaded unless this module is removed.
(This seems to be solved in 2.2.7 or later kernel -- iwai)