Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1 | NOTES ON KERNEL OSS-EMULATION |
| 2 | ============================= |
| 3 | |
| 4 | Jan. 22, 2004 Takashi Iwai <tiwai@suse.de> |
| 5 | |
| 6 | |
| 7 | Modules |
| 8 | ======= |
| 9 | |
| 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. |
| 15 | |
| 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. |
| 20 | |
| 21 | Only necessary step for auto-loading of OSS modules is to define the |
Lucas De Marchi | 970e248 | 2012-03-30 13:37:16 -0700 | [diff] [blame] | 22 | card alias in /etc/modprobe.d/alsa.conf, such as |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 23 | |
| 24 | alias sound-slot-0 snd-emu10k1 |
| 25 | |
| 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 |
| 29 | modutils). |
| 30 | |
| 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. |
| 35 | |
| 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. |
| 39 | |
| 40 | |
| 41 | Device Mapping |
| 42 | ============== |
| 43 | |
| 44 | ALSA supports the following OSS device files: |
| 45 | |
| 46 | PCM: |
| 47 | /dev/dspX |
| 48 | /dev/adspX |
| 49 | |
| 50 | Mixer: |
| 51 | /dev/mixerX |
| 52 | |
| 53 | MIDI: |
| 54 | /dev/midi0X |
| 55 | /dev/amidi0X |
| 56 | |
| 57 | Sequencer: |
| 58 | /dev/sequencer |
| 59 | /dev/sequencer2 (aka /dev/music) |
| 60 | |
| 61 | where X is the card number from 0 to 7. |
| 62 | |
| 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.) |
| 66 | |
| 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. |
| 70 | |
| 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. |
| 75 | |
| 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: |
| 79 | |
| 80 | dsp_map PCM device number assigned to /dev/dspX |
| 81 | (default = 0) |
| 82 | adsp_map PCM device number assigned to /dev/adspX |
| 83 | (default = 1) |
| 84 | |
| 85 | For example, to map the third PCM device (hw:0,2) to /dev/adsp0, |
| 86 | define like this: |
| 87 | |
| 88 | options snd-pcm-oss adsp_map=2 |
| 89 | |
| 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: |
| 93 | |
| 94 | options snd-pcm-oss adsp_map=0,2 |
| 95 | |
| 96 | To change the mapping of MIDI devices, the following options are |
| 97 | available for snd-rawmidi: |
| 98 | |
| 99 | midi_map MIDI device number assigned to /dev/midi0X |
| 100 | (default = 0) |
| 101 | amidi_map MIDI device number assigned to /dev/amidi0X |
| 102 | (default = 1) |
| 103 | |
| 104 | For example, to assign the third MIDI device on the first card to |
| 105 | /dev/midi00, define as follows: |
| 106 | |
| 107 | options snd-rawmidi midi_map=2 |
| 108 | |
| 109 | |
| 110 | PCM Mode |
| 111 | ======== |
| 112 | |
| 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. |
| 118 | |
| 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. |
| 125 | |
| 126 | The command sequence has the following syntax: |
| 127 | |
| 128 | app_name fragments fragment_size [options] |
| 129 | |
| 130 | app_name is the name of application with (higher priority) or without |
| 131 | path. |
| 132 | fragments specifies the number of fragments or zero if no specific |
| 133 | number is given. |
| 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 |
| 136 | available: |
| 137 | |
| 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 |
| 145 | |
| 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: |
| 153 | |
| 154 | % echo "quake 0 0 direct" > /proc/asound/card0/pcm0p/oss |
| 155 | |
| 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: |
| 158 | |
| 159 | % echo "quake 0 0 disable" > /proc/asound/card0/pcm0c/oss |
| 160 | |
| 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. |
| 164 | |
| 165 | The block and non-block options are used to change the behavior of |
| 166 | opening the device file. |
| 167 | |
| 168 | As default, ALSA behaves as original OSS drivers, i.e. does not block |
| 169 | the file when it's busy. The -EBUSY error is returned in this case. |
| 170 | |
| 171 | This blocking behavior can be changed globally via nonblock_open |
| 172 | module option of snd-pcm-oss. For using the blocking mode as default |
| 173 | for OSS devices, define like the following: |
| 174 | |
| 175 | options snd-pcm-oss nonblock_open=0 |
| 176 | |
| 177 | The partial-frag and no-silence commands have been added recently. |
| 178 | Both commands are for optimization use only. The former command |
| 179 | specifies to invoke the write transfer only when the whole fragment is |
| 180 | filled. The latter stops writing the silence data ahead |
| 181 | automatically. Both are disabled as default. |
| 182 | |
| 183 | You can check the currently defined configuration by reading the proc |
| 184 | file. The read image can be sent to the proc file again, hence you |
| 185 | can save the current configuration |
| 186 | |
| 187 | % cat /proc/asound/card0/pcm0p/oss > /somewhere/oss-cfg |
| 188 | |
| 189 | and restore it like |
| 190 | |
| 191 | % cat /somewhere/oss-cfg > /proc/asound/card0/pcm0p/oss |
| 192 | |
| 193 | Also, for clearing all the current configuration, send "erase" command |
| 194 | as below: |
| 195 | |
| 196 | % echo "erase" > /proc/asound/card0/pcm0p/oss |
| 197 | |
| 198 | |
| 199 | Mixer Elements |
| 200 | ============== |
| 201 | |
| 202 | Since ALSA has completely different mixer interface, the emulation of |
| 203 | OSS mixer is relatively complicated. ALSA builds up a mixer element |
| 204 | from several different ALSA (mixer) controls based on the name |
| 205 | string. For example, the volume element SOUND_MIXER_PCM is composed |
| 206 | from "PCM Playback Volume" and "PCM Playback Switch" controls for the |
| 207 | playback direction and from "PCM Capture Volume" and "PCM Capture |
| 208 | Switch" for the capture directory (if exists). When the PCM volume of |
| 209 | OSS is changed, all the volume and switch controls above are adjusted |
| 210 | automatically. |
| 211 | |
| 212 | As default, ALSA uses the following control for OSS volumes: |
| 213 | |
| 214 | OSS volume ALSA control Index |
| 215 | ----------------------------------------------------- |
| 216 | SOUND_MIXER_VOLUME Master 0 |
| 217 | SOUND_MIXER_BASS Tone Control - Bass 0 |
| 218 | SOUND_MIXER_TREBLE Tone Control - Treble 0 |
| 219 | SOUND_MIXER_SYNTH Synth 0 |
| 220 | SOUND_MIXER_PCM PCM 0 |
| 221 | SOUND_MIXER_SPEAKER PC Speaker 0 |
| 222 | SOUND_MIXER_LINE Line 0 |
| 223 | SOUND_MIXER_MIC Mic 0 |
| 224 | SOUND_MIXER_CD CD 0 |
| 225 | SOUND_MIXER_IMIX Monitor Mix 0 |
| 226 | SOUND_MIXER_ALTPCM PCM 1 |
| 227 | SOUND_MIXER_RECLEV (not assigned) |
| 228 | SOUND_MIXER_IGAIN Capture 0 |
| 229 | SOUND_MIXER_OGAIN Playback 0 |
| 230 | SOUND_MIXER_LINE1 Aux 0 |
| 231 | SOUND_MIXER_LINE2 Aux 1 |
| 232 | SOUND_MIXER_LINE3 Aux 2 |
| 233 | SOUND_MIXER_DIGITAL1 Digital 0 |
| 234 | SOUND_MIXER_DIGITAL2 Digital 1 |
| 235 | SOUND_MIXER_DIGITAL3 Digital 2 |
| 236 | SOUND_MIXER_PHONEIN Phone 0 |
| 237 | SOUND_MIXER_PHONEOUT Phone 1 |
| 238 | SOUND_MIXER_VIDEO Video 0 |
| 239 | SOUND_MIXER_RADIO Radio 0 |
| 240 | SOUND_MIXER_MONITOR Monitor 0 |
| 241 | |
| 242 | The second column is the base-string of the corresponding ALSA |
| 243 | control. In fact, the controls with "XXX [Playback|Capture] |
| 244 | [Volume|Switch]" will be checked in addition. |
| 245 | |
| 246 | The current assignment of these mixer elements is listed in the proc |
| 247 | file, /proc/asound/cardX/oss_mixer, which will be like the following |
| 248 | |
| 249 | VOLUME "Master" 0 |
| 250 | BASS "" 0 |
| 251 | TREBLE "" 0 |
| 252 | SYNTH "" 0 |
| 253 | PCM "PCM" 0 |
| 254 | ... |
| 255 | |
| 256 | where the first column is the OSS volume element, the second column |
| 257 | the base-string of the corresponding ALSA control, and the third the |
| 258 | control index. When the string is empty, it means that the |
| 259 | corresponding OSS control is not available. |
| 260 | |
| 261 | For changing the assignment, you can write the configuration to this |
| 262 | proc file. For example, to map "Wave Playback" to the PCM volume, |
| 263 | send the command like the following: |
| 264 | |
| 265 | % echo 'VOLUME "Wave Playback" 0' > /proc/asound/card0/oss_mixer |
| 266 | |
| 267 | The command is exactly as same as listed in the proc file. You can |
| 268 | change one or more elements, one volume per line. In the last |
| 269 | example, both "Wave Playback Volume" and "Wave Playback Switch" will |
| 270 | be affected when PCM volume is changed. |
| 271 | |
| 272 | Like the case of PCM proc file, the permission of proc files depend on |
| 273 | the module options of snd. you'll likely need to be superuser for |
| 274 | sending the command above. |
| 275 | |
| 276 | As well as in the case of PCM proc file, you can save and restore the |
| 277 | current mixer configuration by reading and writing the whole file |
| 278 | image. |
| 279 | |
| 280 | |
Alan Horstmann | 1919de0 | 2007-06-04 23:11:23 +0200 | [diff] [blame] | 281 | Duplex Streams |
| 282 | ============== |
| 283 | |
| 284 | Note that when attempting to use a single device file for playback and |
| 285 | capture, the OSS API provides no way to set the format, sample rate or |
| 286 | number of channels different in each direction. Thus |
| 287 | io_handle = open("device", O_RDWR) |
| 288 | will only function correctly if the values are the same in each direction. |
| 289 | |
| 290 | To use different values in the two directions, use both |
| 291 | input_handle = open("device", O_RDONLY) |
| 292 | output_handle = open("device", O_WRONLY) |
| 293 | and set the values for the corresponding handle. |
| 294 | |
| 295 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 296 | Unsupported Features |
| 297 | ==================== |
| 298 | |
| 299 | MMAP on ICE1712 driver |
| 300 | ---------------------- |
| 301 | ICE1712 supports only the unconventional format, interleaved |
| 302 | 10-channels 24bit (packed in 32bit) format. Therefore you cannot mmap |
| 303 | the buffer as the conventional (mono or 2-channels, 8 or 16bit) format |
| 304 | on OSS. |
| 305 | |