1 NOTES ON KERNEL OSS-EMULATION
2 =============================
4 Jan. 22, 2004 Takashi Iwai <tiwai@suse.de>
10 ALSA provides a powerful OSS emulation on the kernel.
11 The OSS emulation for PCM, mixer and sequencer devices is implemented
12 as add-on kernel modules, snd-pcm-oss, snd-mixer-oss and snd-seq-oss.
13 When you need to access the OSS PCM, mixer or sequencer devices, the
14 corresponding module has to be loaded.
16 These modules are loaded automatically when the corresponding service
17 is called. The alias is defined sound-service-x-y, where x and y are
18 the card number and the minor unit number. Usually you don't have to
19 define these aliases by yourself.
21 Only necessary step for auto-loading of OSS modules is to define the
22 card alias in /etc/modprobe.conf, such as
24 alias sound-slot-0 snd-emu10k1
26 As the second card, define sound-slot-1 as well.
27 Note that you can't use the aliased name as the target name (i.e.
28 "alias sound-slot-0 snd-card-0" doesn't work any more like the old
31 The currently available OSS configuration is shown in
32 /proc/asound/oss/sndstat. This shows in the same syntax of
33 /dev/sndstat, which is available on the commercial OSS driver.
34 On ALSA, you can symlink /dev/sndstat to this proc file.
36 Please note that the devices listed in this proc file appear only
37 after the corresponding OSS-emulation module is loaded. Don't worry
38 even if "NOT ENABLED IN CONFIG" is shown in it.
44 ALSA supports the following OSS device files:
59 /dev/sequencer2 (aka /dev/music)
61 where X is the card number from 0 to 7.
63 (NOTE: Some distributions have the device files like /dev/midi0 and
64 /dev/midi1. They are NOT for OSS but for tclmidi, which is
65 a totally different thing.)
67 Unlike the real OSS, ALSA cannot use the device files more than the
68 assigned ones. For example, the first card cannot use /dev/dsp1 or
69 /dev/dsp2, but only /dev/dsp0 and /dev/adsp0.
71 As seen above, PCM and MIDI may have two devices. Usually, the first
72 PCM device (hw:0,0 in ALSA) is mapped to /dev/dsp and the secondary
73 device (hw:0,1) to /dev/adsp (if available). For MIDI, /dev/midi and
74 /dev/amidi, respectively.
76 You can change this device mapping via the module options of
77 snd-pcm-oss and snd-rawmidi. In the case of PCM, the following
78 options are available for snd-pcm-oss:
80 dsp_map PCM device number assigned to /dev/dspX
82 adsp_map PCM device number assigned to /dev/adspX
85 For example, to map the third PCM device (hw:0,2) to /dev/adsp0,
88 options snd-pcm-oss adsp_map=2
90 The options take arrays. For configuring the second card, specify
91 two entries separated by comma. For example, to map the third PCM
92 device on the second card to /dev/adsp1, define like below:
94 options snd-pcm-oss adsp_map=0,2
96 To change the mapping of MIDI devices, the following options are
97 available for snd-rawmidi:
99 midi_map MIDI device number assigned to /dev/midi0X
101 amidi_map MIDI device number assigned to /dev/amidi0X
104 For example, to assign the third MIDI device on the first card to
105 /dev/midi00, define as follows:
107 options snd-rawmidi midi_map=2
113 As default, ALSA emulates the OSS PCM with so-called plugin layer,
114 i.e. tries to convert the sample format, rate or channels
115 automatically when the card doesn't support it natively.
116 This will lead to some problems for some applications like quake or
117 wine, especially if they use the card only in the MMAP mode.
119 In such a case, you can change the behavior of PCM per application by
120 writing a command to the proc file. There is a proc file for each PCM
121 stream, /proc/asound/cardX/pcmY[cp]/oss, where X is the card number
122 (zero-based), Y the PCM device number (zero-based), and 'p' is for
123 playback and 'c' for capture, respectively. Note that this proc file
124 exists only after snd-pcm-oss module is loaded.
126 The command sequence has the following syntax:
128 app_name fragments fragment_size [options]
130 app_name is the name of application with (higher priority) or without
132 fragments specifies the number of fragments or zero if no specific
134 fragment_size is the size of fragment in bytes or zero if not given.
135 options is the optional parameters. The following options are
138 disable the application tries to open a pcm device for
139 this channel but does not want to use it.
140 direct don't use plugins
141 block force block open mode
142 non-block force non-block open mode
143 partial-frag write also partial fragments (affects playback only)
144 no-silence do not fill silence ahead to avoid clicks
146 The disable option is useful when one stream direction (playback or
147 capture) is not handled correctly by the application although the
148 hardware itself does support both directions.
149 The direct option is used, as mentioned above, to bypass the automatic
150 conversion and useful for MMAP-applications.
151 For example, to playback the first PCM device without plugins for
152 quake, send a command via echo like the following:
154 % echo "quake 0 0 direct" > /proc/asound/card0/pcm0p/oss
156 While quake wants only playback, you may append the second command
157 to notify driver that only this direction is about to be allocated:
159 % echo "quake 0 0 disable" > /proc/asound/card0/pcm0c/oss
161 The permission of proc files depend on the module options of snd.
162 As default it's set as root, so you'll likely need to be superuser for
163 sending the command above.
165 The block and non-block options are used to change the behavior of
166 opening the device file.
167 As default, ALSA behaves as defined in POSIX, i.e. blocks the file
168 when it's busy until the device becomes free (unless O_NONBLOCK is
169 specified). Some applications assume the non-block open behavior,
170 which are actually implemented in some real OSS drivers.
172 This blocking behavior can be changed globally via nonblock_open
173 module option of snd-pcm-oss. For using the non-block mode as default
174 for OSS devices, define like the following:
176 options snd-pcm-oss nonblock_open=1
178 The partial-frag and no-silence commands have been added recently.
179 Both commands are for optimization use only. The former command
180 specifies to invoke the write transfer only when the whole fragment is
181 filled. The latter stops writing the silence data ahead
182 automatically. Both are disabled as default.
184 You can check the currently defined configuration by reading the proc
185 file. The read image can be sent to the proc file again, hence you
186 can save the current configuration
188 % cat /proc/asound/card0/pcm0p/oss > /somewhere/oss-cfg
192 % cat /somewhere/oss-cfg > /proc/asound/card0/pcm0p/oss
194 Also, for clearing all the current configuration, send "erase" command
197 % echo "erase" > /proc/asound/card0/pcm0p/oss
203 Since ALSA has completely different mixer interface, the emulation of
204 OSS mixer is relatively complicated. ALSA builds up a mixer element
205 from several different ALSA (mixer) controls based on the name
206 string. For example, the volume element SOUND_MIXER_PCM is composed
207 from "PCM Playback Volume" and "PCM Playback Switch" controls for the
208 playback direction and from "PCM Capture Volume" and "PCM Capture
209 Switch" for the capture directory (if exists). When the PCM volume of
210 OSS is changed, all the volume and switch controls above are adjusted
213 As default, ALSA uses the following control for OSS volumes:
215 OSS volume ALSA control Index
216 -----------------------------------------------------
217 SOUND_MIXER_VOLUME Master 0
218 SOUND_MIXER_BASS Tone Control - Bass 0
219 SOUND_MIXER_TREBLE Tone Control - Treble 0
220 SOUND_MIXER_SYNTH Synth 0
221 SOUND_MIXER_PCM PCM 0
222 SOUND_MIXER_SPEAKER PC Speaker 0
223 SOUND_MIXER_LINE Line 0
224 SOUND_MIXER_MIC Mic 0
226 SOUND_MIXER_IMIX Monitor Mix 0
227 SOUND_MIXER_ALTPCM PCM 1
228 SOUND_MIXER_RECLEV (not assigned)
229 SOUND_MIXER_IGAIN Capture 0
230 SOUND_MIXER_OGAIN Playback 0
231 SOUND_MIXER_LINE1 Aux 0
232 SOUND_MIXER_LINE2 Aux 1
233 SOUND_MIXER_LINE3 Aux 2
234 SOUND_MIXER_DIGITAL1 Digital 0
235 SOUND_MIXER_DIGITAL2 Digital 1
236 SOUND_MIXER_DIGITAL3 Digital 2
237 SOUND_MIXER_PHONEIN Phone 0
238 SOUND_MIXER_PHONEOUT Phone 1
239 SOUND_MIXER_VIDEO Video 0
240 SOUND_MIXER_RADIO Radio 0
241 SOUND_MIXER_MONITOR Monitor 0
243 The second column is the base-string of the corresponding ALSA
244 control. In fact, the controls with "XXX [Playback|Capture]
245 [Volume|Switch]" will be checked in addition.
247 The current assignment of these mixer elements is listed in the proc
248 file, /proc/asound/cardX/oss_mixer, which will be like the following
257 where the first column is the OSS volume element, the second column
258 the base-string of the corresponding ALSA control, and the third the
259 control index. When the string is empty, it means that the
260 corresponding OSS control is not available.
262 For changing the assignment, you can write the configuration to this
263 proc file. For example, to map "Wave Playback" to the PCM volume,
264 send the command like the following:
266 % echo 'VOLUME "Wave Playback" 0' > /proc/asound/card0/oss_mixer
268 The command is exactly as same as listed in the proc file. You can
269 change one or more elements, one volume per line. In the last
270 example, both "Wave Playback Volume" and "Wave Playback Switch" will
271 be affected when PCM volume is changed.
273 Like the case of PCM proc file, the permission of proc files depend on
274 the module options of snd. you'll likely need to be superuser for
275 sending the command above.
277 As well as in the case of PCM proc file, you can save and restore the
278 current mixer configuration by reading and writing the whole file
285 MMAP on ICE1712 driver
286 ----------------------
287 ICE1712 supports only the unconventional format, interleaved
288 10-channels 24bit (packed in 32bit) format. Therefore you cannot mmap
289 the buffer as the conventional (mono or 2-channels, 8 or 16bit) format
294 Some USB devices support only 24bit format packed in 3bytes. This
295 format is not supported by OSS and no conversion is provided by kernel
296 OSS emulation. You can use the user-space OSS emulation via libaoss