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Documentation / sound / alsa / OSS-Emulation.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

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
22	card alias in /etc/modprobe.d/alsa.conf, such as
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	
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	
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.
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