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- NOTES ON KERNEL OSS-EMULATION
- =============================
-
- Jan. 22, 2004 Takashi Iwai <tiwai@suse.de>
-
-
-Modules
-=======
-
-ALSA provides a powerful OSS emulation on the kernel.
-The OSS emulation for PCM, mixer and sequencer devices is implemented
-as add-on kernel modules, snd-pcm-oss, snd-mixer-oss and snd-seq-oss.
-When you need to access the OSS PCM, mixer or sequencer devices, the
-corresponding module has to be loaded.
-
-These modules are loaded automatically when the corresponding service
-is called. The alias is defined sound-service-x-y, where x and y are
-the card number and the minor unit number. Usually you don't have to
-define these aliases by yourself.
-
-Only necessary step for auto-loading of OSS modules is to define the
-card alias in /etc/modprobe.d/alsa.conf, such as
-
- alias sound-slot-0 snd-emu10k1
-
-As the second card, define sound-slot-1 as well.
-Note that you can't use the aliased name as the target name (i.e.
-"alias sound-slot-0 snd-card-0" doesn't work any more like the old
-modutils).
-
-The currently available OSS configuration is shown in
-/proc/asound/oss/sndstat. This shows in the same syntax of
-/dev/sndstat, which is available on the commercial OSS driver.
-On ALSA, you can symlink /dev/sndstat to this proc file.
-
-Please note that the devices listed in this proc file appear only
-after the corresponding OSS-emulation module is loaded. Don't worry
-even if "NOT ENABLED IN CONFIG" is shown in it.
-
-
-Device Mapping
-==============
-
-ALSA supports the following OSS device files:
-
- PCM:
- /dev/dspX
- /dev/adspX
-
- Mixer:
- /dev/mixerX
-
- MIDI:
- /dev/midi0X
- /dev/amidi0X
-
- Sequencer:
- /dev/sequencer
- /dev/sequencer2 (aka /dev/music)
-
-where X is the card number from 0 to 7.
-
-(NOTE: Some distributions have the device files like /dev/midi0 and
- /dev/midi1. They are NOT for OSS but for tclmidi, which is
- a totally different thing.)
-
-Unlike the real OSS, ALSA cannot use the device files more than the
-assigned ones. For example, the first card cannot use /dev/dsp1 or
-/dev/dsp2, but only /dev/dsp0 and /dev/adsp0.
-
-As seen above, PCM and MIDI may have two devices. Usually, the first
-PCM device (hw:0,0 in ALSA) is mapped to /dev/dsp and the secondary
-device (hw:0,1) to /dev/adsp (if available). For MIDI, /dev/midi and
-/dev/amidi, respectively.
-
-You can change this device mapping via the module options of
-snd-pcm-oss and snd-rawmidi. In the case of PCM, the following
-options are available for snd-pcm-oss:
-
- dsp_map PCM device number assigned to /dev/dspX
- (default = 0)
- adsp_map PCM device number assigned to /dev/adspX
- (default = 1)
-
-For example, to map the third PCM device (hw:0,2) to /dev/adsp0,
-define like this:
-
- options snd-pcm-oss adsp_map=2
-
-The options take arrays. For configuring the second card, specify
-two entries separated by comma. For example, to map the third PCM
-device on the second card to /dev/adsp1, define like below:
-
- options snd-pcm-oss adsp_map=0,2
-
-To change the mapping of MIDI devices, the following options are
-available for snd-rawmidi:
-
- midi_map MIDI device number assigned to /dev/midi0X
- (default = 0)
- amidi_map MIDI device number assigned to /dev/amidi0X
- (default = 1)
-
-For example, to assign the third MIDI device on the first card to
-/dev/midi00, define as follows:
-
- options snd-rawmidi midi_map=2
-
-
-PCM Mode
-========
-
-As default, ALSA emulates the OSS PCM with so-called plugin layer,
-i.e. tries to convert the sample format, rate or channels
-automatically when the card doesn't support it natively.
-This will lead to some problems for some applications like quake or
-wine, especially if they use the card only in the MMAP mode.
-
-In such a case, you can change the behavior of PCM per application by
-writing a command to the proc file. There is a proc file for each PCM
-stream, /proc/asound/cardX/pcmY[cp]/oss, where X is the card number
-(zero-based), Y the PCM device number (zero-based), and 'p' is for
-playback and 'c' for capture, respectively. Note that this proc file
-exists only after snd-pcm-oss module is loaded.
-
-The command sequence has the following syntax:
-
- app_name fragments fragment_size [options]
-
-app_name is the name of application with (higher priority) or without
-path.
-fragments specifies the number of fragments or zero if no specific
-number is given.
-fragment_size is the size of fragment in bytes or zero if not given.
-options is the optional parameters. The following options are
-available:
-
- disable the application tries to open a pcm device for
- this channel but does not want to use it.
- direct don't use plugins
- block force block open mode
- non-block force non-block open mode
- partial-frag write also partial fragments (affects playback only)
- no-silence do not fill silence ahead to avoid clicks
-
-The disable option is useful when one stream direction (playback or
-capture) is not handled correctly by the application although the
-hardware itself does support both directions.
-The direct option is used, as mentioned above, to bypass the automatic
-conversion and useful for MMAP-applications.
-For example, to playback the first PCM device without plugins for
-quake, send a command via echo like the following:
-
- % echo "quake 0 0 direct" > /proc/asound/card0/pcm0p/oss
-
-While quake wants only playback, you may append the second command
-to notify driver that only this direction is about to be allocated:
-
- % echo "quake 0 0 disable" > /proc/asound/card0/pcm0c/oss
-
-The permission of proc files depend on the module options of snd.
-As default it's set as root, so you'll likely need to be superuser for
-sending the command above.
-
-The block and non-block options are used to change the behavior of
-opening the device file.
-
-As default, ALSA behaves as original OSS drivers, i.e. does not block
-the file when it's busy. The -EBUSY error is returned in this case.
-
-This blocking behavior can be changed globally via nonblock_open
-module option of snd-pcm-oss. For using the blocking mode as default
-for OSS devices, define like the following:
-
- options snd-pcm-oss nonblock_open=0
-
-The partial-frag and no-silence commands have been added recently.
-Both commands are for optimization use only. The former command
-specifies to invoke the write transfer only when the whole fragment is
-filled. The latter stops writing the silence data ahead
-automatically. Both are disabled as default.
-
-You can check the currently defined configuration by reading the proc
-file. The read image can be sent to the proc file again, hence you
-can save the current configuration
-
- % cat /proc/asound/card0/pcm0p/oss > /somewhere/oss-cfg
-
-and restore it like
-
- % cat /somewhere/oss-cfg > /proc/asound/card0/pcm0p/oss
-
-Also, for clearing all the current configuration, send "erase" command
-as below:
-
- % echo "erase" > /proc/asound/card0/pcm0p/oss
-
-
-Mixer Elements
-==============
-
-Since ALSA has completely different mixer interface, the emulation of
-OSS mixer is relatively complicated. ALSA builds up a mixer element
-from several different ALSA (mixer) controls based on the name
-string. For example, the volume element SOUND_MIXER_PCM is composed
-from "PCM Playback Volume" and "PCM Playback Switch" controls for the
-playback direction and from "PCM Capture Volume" and "PCM Capture
-Switch" for the capture directory (if exists). When the PCM volume of
-OSS is changed, all the volume and switch controls above are adjusted
-automatically.
-
-As default, ALSA uses the following control for OSS volumes:
-
- OSS volume ALSA control Index
- -----------------------------------------------------
- SOUND_MIXER_VOLUME Master 0
- SOUND_MIXER_BASS Tone Control - Bass 0
- SOUND_MIXER_TREBLE Tone Control - Treble 0
- SOUND_MIXER_SYNTH Synth 0
- SOUND_MIXER_PCM PCM 0
- SOUND_MIXER_SPEAKER PC Speaker 0
- SOUND_MIXER_LINE Line 0
- SOUND_MIXER_MIC Mic 0
- SOUND_MIXER_CD CD 0
- SOUND_MIXER_IMIX Monitor Mix 0
- SOUND_MIXER_ALTPCM PCM 1
- SOUND_MIXER_RECLEV (not assigned)
- SOUND_MIXER_IGAIN Capture 0
- SOUND_MIXER_OGAIN Playback 0
- SOUND_MIXER_LINE1 Aux 0
- SOUND_MIXER_LINE2 Aux 1
- SOUND_MIXER_LINE3 Aux 2
- SOUND_MIXER_DIGITAL1 Digital 0
- SOUND_MIXER_DIGITAL2 Digital 1
- SOUND_MIXER_DIGITAL3 Digital 2
- SOUND_MIXER_PHONEIN Phone 0
- SOUND_MIXER_PHONEOUT Phone 1
- SOUND_MIXER_VIDEO Video 0
- SOUND_MIXER_RADIO Radio 0
- SOUND_MIXER_MONITOR Monitor 0
-
-The second column is the base-string of the corresponding ALSA
-control. In fact, the controls with "XXX [Playback|Capture]
-[Volume|Switch]" will be checked in addition.
-
-The current assignment of these mixer elements is listed in the proc
-file, /proc/asound/cardX/oss_mixer, which will be like the following
-
- VOLUME "Master" 0
- BASS "" 0
- TREBLE "" 0
- SYNTH "" 0
- PCM "PCM" 0
- ...
-
-where the first column is the OSS volume element, the second column
-the base-string of the corresponding ALSA control, and the third the
-control index. When the string is empty, it means that the
-corresponding OSS control is not available.
-
-For changing the assignment, you can write the configuration to this
-proc file. For example, to map "Wave Playback" to the PCM volume,
-send the command like the following:
-
- % echo 'VOLUME "Wave Playback" 0' > /proc/asound/card0/oss_mixer
-
-The command is exactly as same as listed in the proc file. You can
-change one or more elements, one volume per line. In the last
-example, both "Wave Playback Volume" and "Wave Playback Switch" will
-be affected when PCM volume is changed.
-
-Like the case of PCM proc file, the permission of proc files depend on
-the module options of snd. you'll likely need to be superuser for
-sending the command above.
-
-As well as in the case of PCM proc file, you can save and restore the
-current mixer configuration by reading and writing the whole file
-image.
-
-
-Duplex Streams
-==============
-
-Note that when attempting to use a single device file for playback and
-capture, the OSS API provides no way to set the format, sample rate or
-number of channels different in each direction. Thus
- io_handle = open("device", O_RDWR)
-will only function correctly if the values are the same in each direction.
-
-To use different values in the two directions, use both
- input_handle = open("device", O_RDONLY)
- output_handle = open("device", O_WRONLY)
-and set the values for the corresponding handle.
-
-
-Unsupported Features
-====================
-
-MMAP on ICE1712 driver
-----------------------
-ICE1712 supports only the unconventional format, interleaved
-10-channels 24bit (packed in 32bit) format. Therefore you cannot mmap
-the buffer as the conventional (mono or 2-channels, 8 or 16bit) format
-on OSS.
-