[alsa-devel] R: loopback snd_pcm_writei

[Giuliano Zannetti - ART S.p.A.]

Hi Jaroslav,
Thank you for the fix. I tried it and I don't have 0-written-frames anymore.
To be sure that the fix works fine, I compared the multi scenario (forward+loopback) with the forward-only scenario.
It works fine in both cases, but sometimes in the multi scenario I have underruns.
Could it depend from the fix? Or the usage of multi itself can bring to underruns?

-----Messaggio originale-----
Da: Jaroslav Kysela <perex at perex.cz> 
Inviato: martedì 9 aprile 2019 10:37
A: Giuliano Zannetti - ART S.p.A. <giuliano.zannetti at artgroup-spa.com>; alsa-devel at alsa-project.org
Oggetto: Re: [alsa-devel] loopback snd_pcm_writei

Dne 01. 04. 19 v 15:51 Giuliano Zannetti - ART S.p.A. napsal(a):
> Hi,
> 
> I have the configuration in the asound.conf above.
> 
> my_plug - - > my_route - - > my_multi - - > slave0 (forward)
>                                       - - > slave1 (loopback)
> 
> I open the plug my_plug in the block mode:
> 
>     writtenFrames = snd_pcm_writei(handle, bufferToWrite, bufferToWriteSize);
> 
> The issue I see is that there are several "writes" with 0 written frames. It seems like the snd_pcm_writei does not hold until data will be written. In general, 0 written frames sounds weird to me, because snd_pcm_writei should block itself if no space is available in the circular buffer right?
> 
> I have tried to fix the following method in pcm_multi.c, as Jaroslav Kysela suggests.
> 
> -static int snd_pcm_multi_may_wait_for_avail_min(snd_pcm_t *pcm, snd_pcm_uframes_t avail ATTRIBUTE_UNUSED)
> +static int snd_pcm_multi_may_wait_for_avail_min(snd_pcm_t *pcm, snd_pcm_uframes_t avail)
> {
>         snd_pcm_multi_t *multi = pcm->private_data;
> -       snd_pcm_t *slave = multi->slaves[multi->master_slave].pcm;
> -       return snd_pcm_may_wait_for_avail_min(slave, snd_pcm_mmap_avail(slave));
> +       unsigned int i;
> +       for (i = 0; i < multi->slaves_count; ++i) {
> +               if (snd_pcm_may_wait_for_avail_min(multi->slaves[i].pcm, avail))
> +                       return 1;
> +       }
> +       return 0;
> }
> 
> But unfortunately the issue has not been resolved.

Hi,

  I did some debugging (it's pretty reproducible) and the real fixes are here:

https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.alsa-project.org%2F%3Fp%3Dalsa-lib.git%3Ba%3Dcommit%3Bh%3D4694a6643dc09c5e41969d33ed98f42278a02d40&data=02%7C01%7Cgiuliano.zannetti%40artgroup-spa.com%7Ca95dd139ed42412885ec08d6bcc684ce%7C53c55efcdafd47099ce8f76299277497%7C0%7C1%7C636903958159477162&sdata=R9T2kwry3yRBJCA440vmGHFf6sTNeqblW67pCYnw1Is%3D&reserved=0
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  Just get the latest alsa-lib and it should work correctly now. Let me know the result.

					Thanks.
						Jaroslav

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