[alsa-devel] my udev rules are breaking my dmixer setup why?

[Takashi Iwai]

At Tue, 11 Nov 2008 11:57:29 +0100,
Jelle de Jong wrote:
> 
> Takashi Iwai wrote:
> > At Tue, 11 Nov 2008 10:43:00 +0100,
> > Jelle de Jong wrote:
> >> Takashi Iwai wrote:
> >>> At Sun, 09 Nov 2008 15:59:03 +0100,
> >>> Jelle de Jong wrote:
> >>>> Hello everybody,
> >>>>
> >>>> I am trying to use udev to get fixed device names depending on the
> >>>> location of the device on the usb bus. The udev rules seems to be
> >>>> working fine, however i cant get dmix to work with the udev rules using
> >>>> only hw dies work. If i remove the udev rules the dmixer works fine.
> >>>>
> >>>> Can somebody look at the attachment, and maybe do some testing, i would
> >>>> highly appreciate it.
> >>> Swapping the card numbers with udev might have a problem because
> >>> the driver itself remembers the card index, and this number isn't
> >>> swapped -- thus you get inconsistency between the device file and
> >>> the internal index number.
> >>>
> >>> BTW, if you just would like to keep the fixed device index for a
> >>> certain usb device path, try my old patch below.
> >>> It adds devpath option to snd-usb-audio, and you can specify the
> >>> usb device path string ("usb-$BUSNAME-$DEVPATH").
> >>>
> >>>
> >>> Takashi
> >> Thank you Takashi for thanking the time to answer the question. I don't
> >> understand. If udev created the card numbers in the first place,
> >> swapping them should not make a difference?
> > 
> > udev just renames a file.
> > But, the index number is stored in the driver itself, and it can be
> > read via an API function.
> > 
> >> and why does the audio work
> >> fine with only hw:x and not when using dmix:x.
> > 
> > Unless you change the device file name, it works.
> > Or, use an id string instead of an index number.  The id string is
> > found in /proc/asound/cards as [xxx].
> > Or, you can simply set up the slots option of snd module or set up
> > the index option of each module.  See ALSA-Configuration.txt.
> > 
> >> How is the internal index
> >> number created?
> > 
> > Either via a module / kernel boot option, or created dynamically
> > via the driver.
> > 
> >> Can I control this index with udev? did I forget
> >> something in my udev rules?
> > 
> > No.  As mentioned, your udev rule just renames the file.
> > 
> >> Custom compiling sound modules is no option, it will break
> >> maintainability of the system in every way, sorry for this not being an
> >> sustainable option.
> > 
> > IMO, changing udev rule could be more fragile than a custom module...
> > 
> >> Almost all devices can be managed with udev rules, that is where the
> >> system is designed for, there are also alsa rules in there, if they
> >> don't work what is wrong then? is it an alsa issue, or udev, what are
> >> the dependencies when alsa uses hardware. How are the /dev/snd/* devices
> >> used and what is the /proc/asound/* for ?
> > 
> > The card index mechanism in ALSA was introduced much before udev
> > was born.  It's just a legacy mechanism, but it's hard to kill without
> > breaking the running system, unfortunately.
> > 
> > Takashi
> 
> When reading the explanations I understand that we are kind of dealing
> with a legacy issue. Is it possible to add some features to alsa to
> still get what I need to be able to always use a fixed usb port for
> audio for one user. For example if i take the content of ../cards it is
> capable of showing the usb path. If I can use this in my .asoundrc all
> problems go away, and no need for udev rules to rename devices. This
> same system is used input devices for Xorg with /dev/input/by-path/

Yeah, it's worth to add a new identification way via a system path.
It'll be an extention and cannot work as is now, though :)

In your case, try to pass "default", "default_1" or "default_2"
to card id instead of index number.

For example, the below should work without an extra ~/.asoundrc

	% aplay -Dplug:dmix:default foo.wav


Takashi


> cat /proc/asound/cards
> 
>  0 [Intel          ]: HDA-Intel - HDA Intel
>                       HDA Intel at 0xf7db8000 irq 16
>  1 [pcsp           ]: PC-Speaker - pcsp
>                       Internal PC-Speaker at port 0x61
>  2 [Em28xx Audio   ]: Empia Em28xx AudEm28xx Audio - Em28xx Audio
>                       Empia Em28xx Audio
>  3 [default        ]: USB-Audio - USB  AUDIO
>                       USB  AUDIO   at usb-0000:00:1d.7-4.1, full speed
>  4 [default_1      ]: USB-Audio - USB  AUDIO
>                       USB  AUDIO   at usb-0000:00:1d.7-4.2, full speed
>  5 [default_2      ]: USB-Audio - USB  AUDIO
>                       USB  AUDIO   at usb-0000:00:1d.7-4.3, full speed
>  6 [default_3      ]: USB-Audio - USB  AUDIO
>                       USB  AUDIO   at usb-0000:00:1d.7-4.4, full speed
> 
> Would it be possible to create something like the below:
> 
> echo 'pcm.!default {
>    type plug
>    slave.pcm dmixer
> }
> pcm.dmixer {
>    type dmix
>    ipc_key 1024
>    slave.pcm hw:path:usb-0000:00:1d.7-4.1
> }
> ctl.dmixer {
>    type hw
>    card path:usb-0000:00:1d.7-4.1
> }
> pcm.dsp {
>     type plug
>     slave.pcm dmixer
> }
> ctl.mixer {
>     type hw
>     card path:usb-0000:00:1d.7-4.1
> }' | tee /home/user0/.asoundrc
> chmod 600 /home/user0/.asoundrc
> chown user0:user0 /home/user0/.asoundrc
> 
> Thanks in advance,
> 
> Jelle
> 
>