[alsa-devel] virtual sound driver

[Liu, George]

I'm still lost looking at the code. I understand the part to register/remove driver and card. 
One thing on my head is what exactly are the functions in snd_pcm_ops do, and what event will trigger them? 

.open	
.close
.ioctl	(use snd_pcm_lib_ioctl implementation, nothing I need to do)
.hw_params
.hw_free
.prepare
.trigger
.pointer


I added printk in each of the function in aloop, and right after loading the module, dmesg reports whole bunch of pcm_ops. 

* The sequence for each pcm seems to be:   Open -> hw_params -> hw_free -> close
* What determines how many times open->hw_params->hw_free->close is going to be called? Number of snd_pcm or substreams?
*Why pcm_trigger_start is being called?
*If I want to capture playback audio, should I capture the substream associated with the trigger (playback)? 

Thanks so much.
George



-----Original Message-----
From: alsa-devel-bounces at alsa-project.org [mailto:alsa-devel-bounces at alsa-project.org] On Behalf Of Takashi Iwai
Sent: Saturday, July 21, 2012 1:28 AM
To: Liu, George
Cc: alsa-devel at alsa-project.org
Subject: Re: [alsa-devel] virtual sound driver

At Fri, 20 Jul 2012 18:05:40 -0700,
Liu, George wrote:
> 
> I'm trying to write a virtual sound driver to capture PCM data.
> How do I modify dummy.c to capture PCM into buffer?
> 
> Does the PCM data ever sent to any of the pcm_op call backs? There's no read/write callbacks?

Did you take a look at aloop driver?


Takashi
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