Re: [PATCH v7 0/2] ALSA: pcm: reinvent the stream synchronization ID API

[Takashi Iwai <tiwai@suse.de>]

On Tue, 25 Jun 2024 19:25:45 +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.
>     
> It seems that pipewire can benefit from this API (disable adaptive
> resampling for perfectly synchronized PCM streams) now.
>     
> Note that the contents of ID is not supposed to be used for direct
> comparison with a specific byte sequence. The "empty" case is when
> all bytes are zero (driver does not offer this information)
> and all other cases must be only used for equal comparison among
> PCM streams (including different sound cards) if they are using
> identical hardware clock.
>     
> Cc: Takashi Sakamoto <takaswie@kernel.org>
> 
> v6->v7:
>   - rename sync_flag to std_sync_id per Takashi's request
> 
> v5->v6:
>   - remove wrong line from comment (suggested by Amadeusz)
> 
> v4->v5:
>   - extend commit message (clarification for ID use) for first patch
>   - mark union snd_pcm_sync_id as deprecated
>   - do not use get_unaligned() - reviewers are confused and the code
>     is removed in the second path (optimization)
> 
> v3->v4:
>   - more code shuffle as suggested by Takashi
>   - remove unused snd_pcm_empty function in the second patch
>   - put back snd_pcm_set_sync documentation
> 
> v2->v3:
>   - fix pcm_sync_empty() function (wrong comparison) [thanks Takashi Sakamoto]
>   - more documentation for snd_pcm_set_sync_per_card (ID composition)
> 
> v1->v2:
>   - remove union usage per Takashi's request
>   - reduce memory usage
>   - use standard ID generation scheme
> 
> Jaroslav Kysela (2):
>   ALSA: pcm: reinvent the stream synchronization ID API
>   ALSA: pcm: optimize and clarify stream synchronization ID API

I merged both patches now to for-next branch.


Thanks!

Takashi