Re: USB disconnect/connect messages

[Takashi Iwai <tiwai@suse.de>]

At Mon, 21 Jul 2003 00:40:32 +0100,
James Courtier-Dutton wrote:
> 
> Hi,
> 
> Is there any way to tell if a USB sound card has been disconnected or not?
 
IIRC, this is not notified to the application.
the app simply would get an error at the further access.
it sounds not bad to notify the status change to the application by
some way.
perhaps we can add a new control element for this purpose.
and after disconnection, only this element survives to communicate
with the application...


> I would like my application to play sound to a usb sound card, and if 
> the usb cable is unplugged, the application pauses, and if the usb cable 
> is plugged in again, the application continues.
> 
> Example: -
> Some in car stereo systems have usb ports.
> I could plug a PC into the usb port, and play sound from the PC to the 
> car speaker system. If the driver switches off the car stereo (e.g. 
> Receives a mobile phone call) I want the PC to pause playback, but when 
> the car stereo switches on again, I want the PC to continue playback.
> The car stereo switching off looks to the PC as though the usb cable has 
> been unplugged.

from the driver side, i see it's difficult to "resume" from the
disconnection to the disconnection state.  you won't get the same
usb_device record after re-plugging.
of course, it would be possible to handle this in the application side
(or possibly in alsa-lib) "as if" it's reconnected, though.


Takashi


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