Re: [PATCH] ALSA: pcm: reinvent the stream synchronization ID API

[Takashi Iwai <tiwai@suse.de>]

On Tue, 30 Apr 2024 18:10:12 +0200,
Jaroslav Kysela wrote:
> 
> Until the commit e11f0f90a626 ("ALSA: pcm: remove SNDRV_PCM_IOCTL1_INFO
> internal command"), there was a possibility to pass information
> about the synchronized streams to the user space. The mentioned
> commit removed blindly the appropriate code with an irrelevant comment.
> 
> The revert may be appropriate, but since this API was lost for several
> years without any complains, it's time to improve it. The hardware
> parameters may change the used stream clock source (e.g. USB hardware)
> so move this synchronization ID to hw_params as read-only field.

What's the benefit of moving to hw_params?  The current inquiry via
ioctl can also follow the dynamic change, no?  I'm not entirely
objecting to the idea, but need to understand better.

> It seems that pipewire can benefit from this API (disable adaptive
> resampling for perfectly synchronized PCM streams) now.

If the support by pipewire is planned, it'd be good to revive the
feature, yes.

> 
> --- a/include/uapi/sound/asound.h
> +++ b/include/uapi/sound/asound.h
(snip)
> @@ -334,6 +334,7 @@ union snd_pcm_sync_id {
>  	unsigned char id[16];
>  	unsigned short id16[8];
>  	unsigned int id32[4];
> +	__u64 id64[2];
>  };

What's the reason for this __u64 addition?  For the easiness of
zero-compare?


thanks,

Takashi