sw_params for a direct-ed(dmix) hw pcm

[Jaroslav Kysela]

Dne 26. 03. 20 v 13:02 Kai Vehmanen napsal(a):
> Hey,
> 
> On Wed, 25 Mar 2020, sylvain.bertrand at gmail.com wrote:
> 
>> On this issue, I am doing something fundamentaly wrong, but I don't see how to
>> do it right.
>>
>> While configuring a pcm, I should not use sw_params if it is a "direct-ed"
>> (direct::dmix) real hw pcm: in pcm_direct.c, snd_pcm_direct_sw_params
>> function is empty and it seems coherent with the fact the real hw pcm is
>> actually shared and was probably already configured.
> 
> how does the problem appear in your program?
> 
> Applications should just use the ALSA PCM API and not have any special
> casing for different types of PCMs (unless the differences show up via the
> public PCM API). If applications started doing plugin specifics, writing
> and deploying new ALSA plugins would become much harder and kind of defeat
> the whole purpose of the plugin API. In case of dmix, the
> pcm.c:snd_pcm_sw_params() should do the right thing and your application
> should get the cached values.

I agree. Also, the snd_pcm_direct_sw_params() does nothing, because the 
sw_params are already cached in the pcm structure (see comment). It means that 
the dmix (direct) plugins operates with those cached values. Just set 
sw_params like for any other PCM handle. The dmix uses those values (if possible).

						Jaroslav


> 
> Br, Kai
> 


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