[alsa-devel] How to use implicit feedback with full duplex?

Daniel Mack zonque at gmail.com
Tue Mar 26 21:06:22 CET 2013

On 24.03.2013 23:49, Eldad Zack wrote:
> Hi Clemens,
> On Wed, 6 Feb 2013, Clemens Ladisch wrote:
>> Eldad Zack wrote:
>>> On Tue, 5 Feb 2013, Clemens Ladisch wrote:
>>>> I thought I'd try to use implicit feedback with my simple audio device:
>>>> [...]
>>>> This works fine when playing something:
>>> ...
>>>> But when I then try to record at the same time, the driver refuses to
>>>> configure the input endpoint (to the only format, which is already set):
>>>> And despite that "alreay in use" check, the input endpoint is affected
>>>> so much that playback breaks.
>>>> Is full duplex supposed to work?  Does it work with other devices?
>>> This is probably a "yes, but" :)
>>> I use my device mostly full duplex, but with jack opening both
>>> playback and capture at the same time.
>>> I assume you are opening two different streams, one for playback and
>>> one for capture.
>> Jack *also* uses two different streams, but it opens them at the same
>> time.
>>> Can you try using jackd -d alsa -d hw:x with the device and see if that
>>> works for you?
>> That works.  This means that there is a race condition in the driver, or
>> that the different open/hw_params/prepare order trips it up.
> Yes, and also on closing (close/hw_free).
> I finally had some time to look into this - patch below adds some 
> checks and with it:
> * Starting playback, waiting, starting capture: capture doesn't start,
>   playback continues without breaking.
> * Starting capture, waiting, starting playback: playback doesn't start,
>   capture breaks - but it is possible to restart the streams afterwards.
>   [On my setup (with xhci), when the streams break I must restart my box
>   to get them to work again with current code].
> * Starting both - no change, works good.
> This is the order the ops get called when both are start at once, if it 
> helps anyone:
> playback	capture
> open
> 		open
> 		hw_params
> 		 set_format
>  		prepare
>  		 set_format
> hw_params
>  set_format
> prepare
>  set_format
> prepare
>  set_format
> 		prepare
> 		 set_format
>  trigger
> 		trigger
> I'll try to figure out why capture breaks next.
> I still quite slow around the code and don't understand some parts of 
> it so this might take me a while.

Thanks for looking into this!

> @Daniel, do you have any hints for this case (capture breaking
> when starting playback)?

Give me some time to catch up and rebuild my FTU setup please. I hope
I'll be able to get back to you by the end of this week.


More information about the Alsa-devel mailing list