[alsa-devel] [PATCH] control, hw, rawmidi: accept control device node as card identifier when opening

Jaroslav Kysela perex at perex.cz
Tue May 12 13:40:21 CEST 2009

On Tue, 12 May 2009, Jaroslav Kysela wrote:

> On Tue, 12 May 2009, Lennart Poettering wrote:
>> Heya!
>> Traditionally low-level ALSA devices are opened with device strings like
>> 'hw:CARD' where CARD is either a numeric index or a card name. This
>> patch introduces a third syntax that allows you to specify the file name
>> of the control device node. The purpose for this is to make ALSA a bit
>> more like other Linux devices and allow implementaiton of udev device
>> symlinks similar to how disks are currently handled, with
>> /dev/disk/by-path/xxxx /dev/disk/by-id/xxxx and so on.
>> With this patch this line:
>>  aplay -f CD -D hw:/dev/snd/controlC0 < /dev/urandom
>> becomes equivalent to this line:
>>  aplay -f CD -D hw:0 < /dev/urandom
>> A more useful example is this:
>>  aplay -f CD -D hw:/dev/snd/by-path/pci-0000:00:1a.7-usb-0:2:1.2
>> (this line requires a a few additional udev rules)
>> This only works for control devices nodes, not for PCM or any other
>> device nodes. Since the control device node is used as 'entry point'
>> when opening PCM devices this is no limitation.
> NAK. Passing control device to open() functions is not a good idea. The 
> snd_card_get_index() function might be extended to allow /dev style argument. 
> Returned card index can be used as argument for hw: devices without this 
> massive change.
> http://git.alsa-project.org/?p=alsa-lib.git;a=commit;h=33ab0b5381c87e151e87e5000964edd868484886

I just noticed that snd_card_get_index() is used to parse "card" 
configuration item when it's string, so control device name can be passed 
instead card identification to all open() functions in alsa-lib with my 
above patch, too. But it's preferred to use direct card index to reduce
control device open/close sequences.


Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.

More information about the Alsa-devel mailing list