[alsa-devel] paravirtualized alsa kernel driver for XEN

Stefano Panella stefano.panella at citrix.com
Wed Mar 21 11:16:43 CET 2012


Thanks for taking the time to answer about this detail.

On 20/03/12 13:10, Clemens Ladisch wrote:
> Stefano Panella wrote:
>> I have been reading the documentation on "Writing an ALSA Driver" and I
>> am still not completely clear on the meaning of the "pointer" callback
>> in the pcm operations.
>> 1) In case of a playback stream, is the pointer referring to wich sample
>> is currently playing on the DAC or to which is it the last frame read by
>> the HW from the alsa memory buffer?
> It's the position of the first frame not yet read from the memory buffer.
> The delay between the DMA and the DAC output would be reported by
> adjusting runtime->delay, but drivers usually do not bother to do this,
> except when this delay becomes rather large because of additional
> queueing, e.g., in the USB driver.
ok, I was wondering about applications like skype, trying to do some 
background noise cancellation, or to eliminate echo when using speakers.
I was thinking in this case the delay should be accurate for Playback 
and Capture as well, or am I wrong?
>> 4) is there any test I could run to check I have implemented correctly
>> the "pointer" callback? Or any application which would need very high
>> "pointer" precision like frame precision?
> PulseAudio.
> Or run mplayer and look at the A-V value in the status line.
I will try these as well.
> Regards,
> Clemens

In case this pv XEN alsa driver will start to work properly, where 
should I post the patches? Who is currently the linux kernel alsa driver 

Thanks again,


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