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1=============================2Notes on Kernel OSS-Emulation3=============================4 5Jan. 22, 2004 Takashi Iwai <tiwai@suse.de>6 7 8Modules9=======10 11ALSA provides a powerful OSS emulation on the kernel.12The OSS emulation for PCM, mixer and sequencer devices is implemented13as add-on kernel modules, snd-pcm-oss, snd-mixer-oss and snd-seq-oss.14When you need to access the OSS PCM, mixer or sequencer devices, the15corresponding module has to be loaded.16 17These modules are loaded automatically when the corresponding service18is called. The alias is defined ``sound-service-x-y``, where x and y are19the card number and the minor unit number. Usually you don't have to20define these aliases by yourself.21 22Only necessary step for auto-loading of OSS modules is to define the23card alias in ``/etc/modprobe.d/alsa.conf``, such as::24 25 alias sound-slot-0 snd-emu10k126 27As the second card, define ``sound-slot-1`` as well.28Note 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 old30modutils).31 32The currently available OSS configuration is shown in33/proc/asound/oss/sndstat. This shows in the same syntax of34/dev/sndstat, which is available on the commercial OSS driver.35On ALSA, you can symlink /dev/sndstat to this proc file.36 37Please note that the devices listed in this proc file appear only38after the corresponding OSS-emulation module is loaded. Don't worry39even if "NOT ENABLED IN CONFIG" is shown in it.40 41 42Device Mapping43==============44 45ALSA supports the following OSS device files:46::47 48 PCM:49 /dev/dspX50 /dev/adspX51 52 Mixer:53 /dev/mixerX54 55 MIDI:56 /dev/midi0X57 /dev/amidi0X58 59 Sequencer:60 /dev/sequencer61 /dev/sequencer2 (aka /dev/music)62 63where X is the card number from 0 to 7.64 65(NOTE: Some distributions have the device files like /dev/midi0 and66/dev/midi1. They are NOT for OSS but for tclmidi, which is67a totally different thing.)68 69Unlike the real OSS, ALSA cannot use the device files more than the70assigned ones. For example, the first card cannot use /dev/dsp1 or71/dev/dsp2, but only /dev/dsp0 and /dev/adsp0.72 73As seen above, PCM and MIDI may have two devices. Usually, the first74PCM device (``hw:0,0`` in ALSA) is mapped to /dev/dsp and the secondary75device (``hw:0,1``) to /dev/adsp (if available). For MIDI, /dev/midi and76/dev/amidi, respectively.77 78You can change this device mapping via the module options of79snd-pcm-oss and snd-rawmidi. In the case of PCM, the following80options are available for snd-pcm-oss:81 82dsp_map83 PCM device number assigned to /dev/dspX84 (default = 0)85adsp_map86 PCM device number assigned to /dev/adspX87 (default = 1)88 89For example, to map the third PCM device (``hw:0,2``) to /dev/adsp0,90define like this:91::92 93 options snd-pcm-oss adsp_map=294 95The options take arrays. For configuring the second card, specify96two entries separated by comma. For example, to map the third PCM97device on the second card to /dev/adsp1, define like below:98::99 100 options snd-pcm-oss adsp_map=0,2101 102To change the mapping of MIDI devices, the following options are103available for snd-rawmidi:104 105midi_map106 MIDI device number assigned to /dev/midi0X107 (default = 0)108amidi_map109 MIDI device number assigned to /dev/amidi0X110 (default = 1)111 112For example, to assign the third MIDI device on the first card to113/dev/midi00, define as follows:114::115 116 options snd-rawmidi midi_map=2117 118 119PCM Mode120========121 122As default, ALSA emulates the OSS PCM with so-called plugin layer,123i.e. tries to convert the sample format, rate or channels124automatically when the card doesn't support it natively.125This will lead to some problems for some applications like quake or126wine, especially if they use the card only in the MMAP mode.127 128In such a case, you can change the behavior of PCM per application by129writing a command to the proc file. There is a proc file for each PCM130stream, ``/proc/asound/cardX/pcmY[cp]/oss``, where X is the card number131(zero-based), Y the PCM device number (zero-based), and ``p`` is for132playback and ``c`` for capture, respectively. Note that this proc file133exists only after snd-pcm-oss module is loaded.134 135The 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 without141path.142``fragments`` specifies the number of fragments or zero if no specific143number 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 are146available:147 148disable149 the application tries to open a pcm device for150 this channel but does not want to use it.151direct152 don't use plugins153block154 force block open mode155non-block156 force non-block open mode157partial-frag158 write also partial fragments (affects playback only)159no-silence160 do not fill silence ahead to avoid clicks161 162The ``disable`` option is useful when one stream direction (playback or163capture) is not handled correctly by the application although the164hardware itself does support both directions.165The ``direct`` option is used, as mentioned above, to bypass the automatic166conversion and useful for MMAP-applications.167For example, to playback the first PCM device without plugins for168quake, send a command via echo like the following:169::170 171 % echo "quake 0 0 direct" > /proc/asound/card0/pcm0p/oss172 173While quake wants only playback, you may append the second command174to notify driver that only this direction is about to be allocated:175::176 177 % echo "quake 0 0 disable" > /proc/asound/card0/pcm0c/oss178 179The permission of proc files depend on the module options of snd.180As default it's set as root, so you'll likely need to be superuser for181sending the command above.182 183The block and non-block options are used to change the behavior of184opening the device file.185 186As default, ALSA behaves as original OSS drivers, i.e. does not block187the file when it's busy. The -EBUSY error is returned in this case.188 189This blocking behavior can be changed globally via nonblock_open190module option of snd-pcm-oss. For using the blocking mode as default191for OSS devices, define like the following:192::193 194 options snd-pcm-oss nonblock_open=0195 196The ``partial-frag`` and ``no-silence`` commands have been added recently.197Both commands are for optimization use only. The former command198specifies to invoke the write transfer only when the whole fragment is199filled. The latter stops writing the silence data ahead200automatically. Both are disabled as default.201 202You can check the currently defined configuration by reading the proc203file. The read image can be sent to the proc file again, hence you204can save the current configuration205::206 207 % cat /proc/asound/card0/pcm0p/oss > /somewhere/oss-cfg208 209and restore it like210::211 212 % cat /somewhere/oss-cfg > /proc/asound/card0/pcm0p/oss213 214Also, for clearing all the current configuration, send ``erase`` command215as below:216::217 218 % echo "erase" > /proc/asound/card0/pcm0p/oss219 220 221Mixer Elements222==============223 224Since ALSA has completely different mixer interface, the emulation of225OSS mixer is relatively complicated. ALSA builds up a mixer element226from several different ALSA (mixer) controls based on the name227string. For example, the volume element SOUND_MIXER_PCM is composed228from "PCM Playback Volume" and "PCM Playback Switch" controls for the229playback direction and from "PCM Capture Volume" and "PCM Capture230Switch" for the capture directory (if exists). When the PCM volume of231OSS is changed, all the volume and switch controls above are adjusted232automatically.233 234As default, ALSA uses the following control for OSS volumes:235 236==================== ===================== =====237OSS volume ALSA control Index238==================== ===================== =====239SOUND_MIXER_VOLUME Master 0240SOUND_MIXER_BASS Tone Control - Bass 0241SOUND_MIXER_TREBLE Tone Control - Treble 0242SOUND_MIXER_SYNTH Synth 0243SOUND_MIXER_PCM PCM 0244SOUND_MIXER_SPEAKER PC Speaker 0245SOUND_MIXER_LINE Line 0246SOUND_MIXER_MIC Mic 0247SOUND_MIXER_CD CD 0248SOUND_MIXER_IMIX Monitor Mix 0249SOUND_MIXER_ALTPCM PCM 1250SOUND_MIXER_RECLEV (not assigned)251SOUND_MIXER_IGAIN Capture 0252SOUND_MIXER_OGAIN Playback 0253SOUND_MIXER_LINE1 Aux 0254SOUND_MIXER_LINE2 Aux 1255SOUND_MIXER_LINE3 Aux 2256SOUND_MIXER_DIGITAL1 Digital 0257SOUND_MIXER_DIGITAL2 Digital 1258SOUND_MIXER_DIGITAL3 Digital 2259SOUND_MIXER_PHONEIN Phone 0260SOUND_MIXER_PHONEOUT Phone 1261SOUND_MIXER_VIDEO Video 0262SOUND_MIXER_RADIO Radio 0263SOUND_MIXER_MONITOR Monitor 0264==================== ===================== =====265 266The second column is the base-string of the corresponding ALSA267control. In fact, the controls with ``XXX [Playback|Capture]268[Volume|Switch]`` will be checked in addition.269 270The current assignment of these mixer elements is listed in the proc271file, /proc/asound/cardX/oss_mixer, which will be like the following272::273 274 VOLUME "Master" 0275 BASS "" 0276 TREBLE "" 0277 SYNTH "" 0278 PCM "PCM" 0279 ...280 281where the first column is the OSS volume element, the second column282the base-string of the corresponding ALSA control, and the third the283control index. When the string is empty, it means that the284corresponding OSS control is not available.285 286For changing the assignment, you can write the configuration to this287proc file. For example, to map "Wave Playback" to the PCM volume,288send the command like the following:289::290 291 % echo 'VOLUME "Wave Playback" 0' > /proc/asound/card0/oss_mixer292 293The command is exactly as same as listed in the proc file. You can294change one or more elements, one volume per line. In the last295example, both "Wave Playback Volume" and "Wave Playback Switch" will296be affected when PCM volume is changed.297 298Like the case of PCM proc file, the permission of proc files depend on299the module options of snd. you'll likely need to be superuser for300sending the command above.301 302As well as in the case of PCM proc file, you can save and restore the303current mixer configuration by reading and writing the whole file304image.305 306 307Duplex Streams308==============309 310Note that when attempting to use a single device file for playback and311capture, the OSS API provides no way to set the format, sample rate or312number of channels different in each direction. Thus313::314 315 io_handle = open("device", O_RDWR)316 317will only function correctly if the values are the same in each direction.318 319To use different values in the two directions, use both320::321 322 input_handle = open("device", O_RDONLY)323 output_handle = open("device", O_WRONLY)324 325and set the values for the corresponding handle.326 327 328Unsupported Features329====================330 331MMAP on ICE1712 driver332----------------------333ICE1712 supports only the unconventional format, interleaved33410-channels 24bit (packed in 32bit) format. Therefore you cannot mmap335the buffer as the conventional (mono or 2-channels, 8 or 16bit) format336on OSS.337