ive got 2 working implementations of the loopback snd-aloop device

ive got 2 working implementations of the loopback snd-aloop device

[Ahmet İnan]

about:
userspace->kernel->userspace
without wasting too much in kernel. no dmixin, no mixin at all and no
format changing is done in kernel. its only a loopback device.
for this to work, it forces the other side to use the same rate, format
and number of channels.

reason:
aoss doesnt always work for me. and imho is not the right way.
LD_PRELOAD'ing is just plain braindamaged, it couses more trouble than it
solves. telling vendors to use alsa also doesnt help.
me wants mixing very bad. me wants network transparency for all apps.

ive just hacked this two implementations, just to find out, if these two
methods would work:

1. ringbuffer-copy:
the first method was very easy to implement and works perfectly, without
fracking too much with alsa internals. it allows different buffer and
period sizes on both sides. but it consumes more cpu cycles for the
memcpy. the capture timer checks if playback is valid and running, then
copies from the playbacks current position to its own buffers current
position.

2. overwriting dma_area with other sides dma_area:
the second method is a little harder to implement and it is still not
finnished, but also works perfectly. not finnished because after "stealing"
the others dma area, it cant simply free both dma buffers:
after letting alsa do all the hard work of allocating and setting up dma,
it simply overwrites the dma_area with the dma_area of the other side.
for this to work, it also forces the other side to use the same buffer
and period sizes. it is very efficient, couse it doesnt need to copy
buffers. on each SNDRV_PCM_TRIGGER_START it synchronizes the buffer
position, to reduce latency.

features:
currently it can have up to 8 simultaneous playback and capture streams
(i hope this is enough to tell you what i mean, also see my asoundrc):

ecasound -B:nonrt -f:16,2,48000 -i:alsahw,0,1,0 &
ecasound -B:nonrt -f:16,2,48000 -i:alsahw,0,1,1 &
ecasound -B:nonrt -f:16,2,48000 -i:alsahw,0,1,2 &
...

or, if ecasound is not to your avail but want to test it:
arecord -D hw:0,1 -t raw -c 2 -r 48000 -f S16_LE | aplay -t raw -c 2 -r 48000 -f S16_LE -

now open up to 8 oss applications _without_ aoss, sit back and relax.

todo:
the second method needs a better way of doing the job. it needs to allocate
dma and do the voodoo stuff and then set both sides to it. this way, after
closing both sides, it can free up again, thus eliminating the memory leak.

its currently only a half duplex loopback but fullduplex device, couse
the missing part is dummy code. it would be no trouble at all to implement
this.

after sorting out problems im going to write an userspace app that will
do what ecasound above does, but transparent to the user. maybe someone
wants to help.

cleaning up.

problems:
i have HZ=1000:
2*2*44100/1000 = 176.4, not 176
48000 works perfect tho. HZ=100 wont show these problems (different story
when using 11025, for example). i tried bresenham to change pcm_jiffies
constantly, but it actually gave me more problems than it solved.
maybe using rtc/hpet with fallback to jiffie would be a better idea.

files:
http://www.mathematik.uni-freiburg.de/IAM/homepages/ainan/aloop-kernel-ringbuffer-copy.c
http://www.mathematik.uni-freiburg.de/IAM/homepages/ainan/aloop-kernel-efficient_but_complex.c
http://www.mathematik.uni-freiburg.de/IAM/homepages/ainan/alsa-driver-1.0.13-aloop-ainan-ringbuffer-copy.diff
http://www.mathematik.uni-freiburg.de/IAM/homepages/ainan/alsa-driver-1.0.13-aloop-ainan-efficient_but_complex.diff

last but not least:
hooray!

ahmet

--
admin der abteilung für angewandte mathematik, tel. 0761-203-5626

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Re: ive got 2 working implementations of the loopback snd-aloop device

[Lee Revell]

On Sun, 2006-12-17 at 13:39 +0100, Ahmet İnan wrote:
> reason:
> aoss doesnt always work for me. and imho is not the right way.
> LD_PRELOAD'ing is just plain braindamaged, it couses more trouble than
> it solves. telling vendors to use alsa also doesnt help.
> me wants mixing very bad. me wants network transparency for all apps.
> 

This seems like a lot of wasted cycles just to make life easier for lazy
application programmers.

Telling vendors to use ALSA does indeed help - the latest releases of
Flash and Skype support ALSA natively.

Lee


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Re: ive got 2 working implementations of the loopback snd-aloop device

[Ahmet İnan]

On Mon, Dec 18, 2006 at 12:47:20PM -0500, Lee Revell wrote:
> On Sun, 2006-12-17 at 13:39 +0100, Ahmet İnan wrote:
> > reason:
> > aoss doesnt always work for me. and imho is not the right way.
> > LD_PRELOAD'ing is just plain braindamaged, it couses more trouble than
> > it solves. telling vendors to use alsa also doesnt help.
> > me wants mixing very bad. me wants network transparency for all apps.
> This seems like a lot of wasted cycles just to make life easier for lazy
> application programmers.
/* the same goes to aoss, which is also a waste of cycles and nerves. */
well comfort comes always with a higher price to pay. and in this respect
i made it easier for my own life. it was the last time this
properitary-but-hard-to-beat app annoyed me couse of this one shortcomming.
so im happy now. felt like sharing. ^_^

> Telling vendors to use ALSA does indeed help - the latest releases of
> Flash and Skype support ALSA natively.
yeah, but it takes them YEARS to react, until the app is outdated or of no
use / interest annymore. its almoust as futile to tell them to make their
apps also work on my favorite os.

so until they do, at least, ppl like me can enjoy things, what manny other
ppl who dont know how it works, already do.

but hey, at least alsa starts to have a working loopback device, again, it
can be used for a lot of nice things.

ahmet


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