1 =============================
2 Notes on Kernel OSS-Emulation
3 =============================
4
5 Jan. 22, 2004 Takashi Iwai <tiwai@suse.de>
6
7
8 Modules
9 =======
10
11 ALSA provides a powerful OSS emulation on the kernel.
12 The OSS emulation for PCM, mixer and sequencer devices is implemented
13 as add-on kernel modules, snd-pcm-oss, snd-mixer-oss and snd-seq-oss.
14 When you need to access the OSS PCM, mixer or sequencer devices, the
15 corresponding module has to be loaded.
16
17 These modules are loaded automatically when the corresponding service
18 is called. The alias is defined ``sound-service-x-y``, where x and y are
19 the card number and the minor unit number. Usually you don't have to
20 define these aliases by yourself.
21
22 Only necessary step for auto-loading of OSS modules is to define the
23 card alias in ``/etc/modprobe.d/alsa.conf``, such as::
24
25 alias sound-slot-0 snd-emu10k1
26
27 As the second card, define ``sound-slot-1`` as well.
28 Note that you can't use the aliased name as the target name (i.e.
29 ``alias sound-slot-0 snd-card-0`` doesn't work any more like the old
30 modutils).
31
32 The currently available OSS configuration is shown in
33 /proc/asound/oss/sndstat. This shows in the same syntax of
34 /dev/sndstat, which is available on the commercial OSS driver.
35 On ALSA, you can symlink /dev/sndstat to this proc file.
36
37 Please note that the devices listed in this proc file appear only
38 after the corresponding OSS-emulation module is loaded. Don't worry
39 even if "NOT ENABLED IN CONFIG" is shown in it.
40
41
42 Device Mapping
43 ==============
44
45 ALSA supports the following OSS device files:
46 ::
47
48 PCM:
49 /dev/dspX
50 /dev/adspX
51
52 Mixer:
53 /dev/mixerX
54
55 MIDI:
56 /dev/midi0X
57 /dev/amidi0X
58
59 Sequencer:
60 /dev/sequencer
61 /dev/sequencer2 (aka /dev/music)
62
63 where X is the card number from 0 to 7.
64
65 (NOTE: Some distributions have the device files like /dev/midi0 and
66 /dev/midi1. They are NOT for OSS but for tclmidi, which is
67 a totally different thing.)
68
69 Unlike the real OSS, ALSA cannot use the device files more than the
70 assigned ones. For example, the first card cannot use /dev/dsp1 or
71 /dev/dsp2, but only /dev/dsp0 and /dev/adsp0.
72
73 As seen above, PCM and MIDI may have two devices. Usually, the first
74 PCM device (``hw:0,0`` in ALSA) is mapped to /dev/dsp and the secondary
75 device (``hw:0,1``) to /dev/adsp (if available). For MIDI, /dev/midi and
76 /dev/amidi, respectively.
77
78 You can change this device mapping via the module options of
79 snd-pcm-oss and snd-rawmidi. In the case of PCM, the following
80 options are available for snd-pcm-oss:
81
82 dsp_map
83 PCM device number assigned to /dev/dspX
84 (default = 0)
85 adsp_map
86 PCM device number assigned to /dev/adspX
87 (default = 1)
88
89 For example, to map the third PCM device (``hw:0,2``) to /dev/adsp0,
90 define like this:
91 ::
92
93 options snd-pcm-oss adsp_map=2
94
95 The options take arrays. For configuring the second card, specify
96 two entries separated by comma. For example, to map the third PCM
97 device on the second card to /dev/adsp1, define like below:
98 ::
99
100 options snd-pcm-oss adsp_map=0,2
101
102 To change the mapping of MIDI devices, the following options are
103 available for snd-rawmidi:
104
105 midi_map
106 MIDI device number assigned to /dev/midi0X
107 (default = 0)
108 amidi_map
109 MIDI device number assigned to /dev/amidi0X
110 (default = 1)
111
112 For example, to assign the third MIDI device on the first card to
113 /dev/midi00, define as follows:
114 ::
115
116 options snd-rawmidi midi_map=2
117
118
119 PCM Mode
120 ========
121
122 As default, ALSA emulates the OSS PCM with so-called plugin layer,
123 i.e. tries to convert the sample format, rate or channels
124 automatically when the card doesn't support it natively.
125 This will lead to some problems for some applications like quake or
126 wine, especially if they use the card only in the MMAP mode.
127
128 In such a case, you can change the behavior of PCM per application by
129 writing a command to the proc file. There is a proc file for each PCM
130 stream, ``/proc/asound/cardX/pcmY[cp]/oss``, where X is the card number
131 (zero-based), Y the PCM device number (zero-based), and ``p`` is for
132 playback and ``c`` for capture, respectively. Note that this proc file
133 exists only after snd-pcm-oss module is loaded.
134
135 The command sequence has the following syntax:
136 ::
137
138 app_name fragments fragment_size [options]
139
140 ``app_name`` is the name of application with (higher priority) or without
141 path.
142 ``fragments`` specifies the number of fragments or zero if no specific
143 number is given.
144 ``fragment_size`` is the size of fragment in bytes or zero if not given.
145 ``options`` is the optional parameters. The following options are
146 available:
147
148 disable
149 the application tries to open a pcm device for
150 this channel but does not want to use it.
151 direct
152 don't use plugins
153 block
154 force block open mode
155 non-block
156 force non-block open mode
157 partial-frag
158 write also partial fragments (affects playback only)
159 no-silence
160 do not fill silence ahead to avoid clicks
161
162 The ``disable`` option is useful when one stream direction (playback or
163 capture) is not handled correctly by the application although the
164 hardware itself does support both directions.
165 The ``direct`` option is used, as mentioned above, to bypass the automatic
166 conversion and useful for MMAP-applications.
167 For example, to playback the first PCM device without plugins for
168 quake, send a command via echo like the following:
169 ::
170
171 % echo "quake 0 0 direct" > /proc/asound/card0/pcm0p/oss
172
173 While quake wants only playback, you may append the second command
174 to notify driver that only this direction is about to be allocated:
175 ::
176
177 % echo "quake 0 0 disable" > /proc/asound/card0/pcm0c/oss
178
179 The permission of proc files depend on the module options of snd.
180 As default it's set as root, so you'll likely need to be superuser for
181 sending the command above.
182
183 The block and non-block options are used to change the behavior of
184 opening the device file.
185
186 As default, ALSA behaves as original OSS drivers, i.e. does not block
187 the file when it's busy. The -EBUSY error is returned in this case.
188
189 This blocking behavior can be changed globally via nonblock_open
190 module option of snd-pcm-oss. For using the blocking mode as default
191 for OSS devices, define like the following:
192 ::
193
194 options snd-pcm-oss nonblock_open=0
195
196 The ``partial-frag`` and ``no-silence`` commands have been added recently.
197 Both commands are for optimization use only. The former command
198 specifies to invoke the write transfer only when the whole fragment is
199 filled. The latter stops writing the silence data ahead
200 automatically. Both are disabled as default.
201
202 You can check the currently defined configuration by reading the proc
203 file. The read image can be sent to the proc file again, hence you
204 can save the current configuration
205 ::
206
207 % cat /proc/asound/card0/pcm0p/oss > /somewhere/oss-cfg
208
209 and restore it like
210 ::
211
212 % cat /somewhere/oss-cfg > /proc/asound/card0/pcm0p/oss
213
214 Also, for clearing all the current configuration, send ``erase`` command
215 as below:
216 ::
217
218 % echo "erase" > /proc/asound/card0/pcm0p/oss
219
220
221 Mixer Elements
222 ==============
223
224 Since ALSA has completely different mixer interface, the emulation of
225 OSS mixer is relatively complicated. ALSA builds up a mixer element
226 from several different ALSA (mixer) controls based on the name
227 string. For example, the volume element SOUND_MIXER_PCM is composed
228 from "PCM Playback Volume" and "PCM Playback Switch" controls for the
229 playback direction and from "PCM Capture Volume" and "PCM Capture
230 Switch" for the capture directory (if exists). When the PCM volume of
231 OSS is changed, all the volume and switch controls above are adjusted
232 automatically.
233
234 As default, ALSA uses the following control for OSS volumes:
235
236 ==================== ===================== =====
237 OSS volume ALSA control Index
238 ==================== ===================== =====
239 SOUND_MIXER_VOLUME Master 0
240 SOUND_MIXER_BASS Tone Control - Bass 0
241 SOUND_MIXER_TREBLE Tone Control - Treble 0
242 SOUND_MIXER_SYNTH Synth 0
243 SOUND_MIXER_PCM PCM 0
244 SOUND_MIXER_SPEAKER PC Speaker 0
245 SOUND_MIXER_LINE Line 0
246 SOUND_MIXER_MIC Mic 0
247 SOUND_MIXER_CD CD 0
248 SOUND_MIXER_IMIX Monitor Mix 0
249 SOUND_MIXER_ALTPCM PCM 1
250 SOUND_MIXER_RECLEV (not assigned)
251 SOUND_MIXER_IGAIN Capture 0
252 SOUND_MIXER_OGAIN Playback 0
253 SOUND_MIXER_LINE1 Aux 0
254 SOUND_MIXER_LINE2 Aux 1
255 SOUND_MIXER_LINE3 Aux 2
256 SOUND_MIXER_DIGITAL1 Digital 0
257 SOUND_MIXER_DIGITAL2 Digital 1
258 SOUND_MIXER_DIGITAL3 Digital 2
259 SOUND_MIXER_PHONEIN Phone 0
260 SOUND_MIXER_PHONEOUT Phone 1
261 SOUND_MIXER_VIDEO Video 0
262 SOUND_MIXER_RADIO Radio 0
263 SOUND_MIXER_MONITOR Monitor 0
264 ==================== ===================== =====
265
266 The second column is the base-string of the corresponding ALSA
267 control. In fact, the controls with ``XXX [Playback|Capture]
268 [Volume|Switch]`` will be checked in addition.
269
270 The current assignment of these mixer elements is listed in the proc
271 file, /proc/asound/cardX/oss_mixer, which will be like the following
272 ::
273
274 VOLUME "Master" 0
275 BASS "" 0
276 TREBLE "" 0
277 SYNTH "" 0
278 PCM "PCM" 0
279 ...
280
281 where the first column is the OSS volume element, the second column
282 the base-string of the corresponding ALSA control, and the third the
283 control index. When the string is empty, it means that the
284 corresponding OSS control is not available.
285
286 For changing the assignment, you can write the configuration to this
287 proc file. For example, to map "Wave Playback" to the PCM volume,
288 send the command like the following:
289 ::
290
291 % echo 'VOLUME "Wave Playback" 0' > /proc/asound/card0/oss_mixer
292
293 The command is exactly as same as listed in the proc file. You can
294 change one or more elements, one volume per line. In the last
295 example, both "Wave Playback Volume" and "Wave Playback Switch" will
296 be affected when PCM volume is changed.
297
298 Like the case of PCM proc file, the permission of proc files depend on
299 the module options of snd. you'll likely need to be superuser for
300 sending the command above.
301
302 As well as in the case of PCM proc file, you can save and restore the
303 current mixer configuration by reading and writing the whole file
304 image.
305
306
307 Duplex Streams
308 ==============
309
310 Note that when attempting to use a single device file for playback and
311 capture, the OSS API provides no way to set the format, sample rate or
312 number of channels different in each direction. Thus
313 ::
314
315 io_handle = open("device", O_RDWR)
316
317 will only function correctly if the values are the same in each direction.
318
319 To use different values in the two directions, use both
320 ::
321
322 input_handle = open("device", O_RDONLY)
323 output_handle = open("device", O_WRONLY)
324
325 and set the values for the corresponding handle.
326
327
328 Unsupported Features
329 ====================
330
331 MMAP on ICE1712 driver
332 ----------------------
333 ICE1712 supports only the unconventional format, interleaved
334 10-channels 24bit (packed in 32bit) format. Therefore you cannot mmap
335 the buffer as the conventional (mono or 2-channels, 8 or 16bit) format
336 on OSS.
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