Re: [PATCH v5 1/2] ALSA: pcm: reinvent the stream synchronization ID API

["Amadeusz Sławiński" <amadeuszx.slawinski@linux.intel.com>]

On 6/24/2024 3:07 PM, 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>
> Signed-off-by: Jaroslav Kysela <perex@perex.cz>
> ---

(...)

>   
> @@ -420,7 +420,8 @@ struct snd_pcm_hw_params {
>   	unsigned int rate_num;		/* R: rate numerator */
>   	unsigned int rate_den;		/* R: rate denominator */
>   	snd_pcm_uframes_t fifo_size;	/* R: chip FIFO size in frames */
> -	unsigned char reserved[64];	/* reserved for future */
> +	unsigned char sync[16];		/* R: synchronization ID (perfect sync - one clock source) */

If it is introduced as new API, can't this be done better? Maybe like:
struct sync {
	char cardnum[4];
	char id[12];
};
or maybe just:
struct sync {
	u32 cardnum;
	char id[12];
};
or something like that? It is bit hard to follow in next patch all this 
params->sync + 4 and memset/strncpy. And having named fields would help.

Thanks,
Amadeusz