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

[Takashi Sakamoto <o-takashi@sakamocchi.jp>]

On Mon, May 06, 2024 at 05:11:49PM +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.
> 
> Cc: Takashi Sakamoto <takaswie@kernel.org>
> Signed-off-by: Jaroslav Kysela <perex@perex.cz>
> ---
>  include/sound/pcm.h         | 11 ++++++++++-
>  include/uapi/sound/asound.h | 13 ++++---------
>  sound/core/pcm_lib.c        | 18 ++++++++++++++----
>  sound/core/pcm_native.c     |  6 ++++++
>  sound/pci/emu10k1/p16v.c    |  7 +++----
>  5 files changed, 37 insertions(+), 18 deletions(-)
> 
> diff --git a/include/sound/pcm.h b/include/sound/pcm.h
> index 210096f124ee..dae41b100517 100644
> --- a/include/sound/pcm.h
> +++ b/include/sound/pcm.h
> @@ -93,6 +93,7 @@ struct snd_pcm_ops {
>  #define SNDRV_PCM_IOCTL1_CHANNEL_INFO	2
>  /* 3 is absent slot. */
>  #define SNDRV_PCM_IOCTL1_FIFO_SIZE	4
> +#define SNDRV_PCM_IOCTL1_SYNC_ID	5
>  
>  #define SNDRV_PCM_TRIGGER_STOP		0
>  #define SNDRV_PCM_TRIGGER_START		1
> @@ -396,7 +397,7 @@ struct snd_pcm_runtime {
>  	snd_pcm_uframes_t silence_start; /* starting pointer to silence area */
>  	snd_pcm_uframes_t silence_filled; /* already filled part of silence area */
>  
> -	union snd_pcm_sync_id sync;	/* hardware synchronization ID */
> +	unsigned char sync[16];		/* hardware synchronization ID */
>  
>  	/* -- mmap -- */
>  	struct snd_pcm_mmap_status *status;
> @@ -1565,6 +1566,14 @@ static inline u64 pcm_format_to_bits(snd_pcm_format_t pcm_format)
>  	     (__force int)(f) <= (__force int)SNDRV_PCM_FORMAT_LAST;	\
>  	     (f) = (__force snd_pcm_format_t)((__force int)(f) + 1))
>  
> +/**
> + * is sync id (clock id) empty?
> + */
> +static inline bool pcm_sync_empty(const unsigned char *sync)
> +{
> +	return strnlen((const char *)sync, 16) == 0;
> +}
> +

For the sound card indexed by 0, it is problematic, since in the later
change, the first element of array has 0.

>  /* printk helpers */
>  #define pcm_err(pcm, fmt, args...) \
>  	dev_err((pcm)->card->dev, fmt, ##args)
> diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h
> index 628d46a0da92..c458172b32d5 100644
> --- a/include/uapi/sound/asound.h
> +++ b/include/uapi/sound/asound.h
> @@ -142,7 +142,7 @@ struct snd_hwdep_dsp_image {
>   *                                                                           *
>   *****************************************************************************/
>  
> -#define SNDRV_PCM_VERSION		SNDRV_PROTOCOL_VERSION(2, 0, 17)
> +#define SNDRV_PCM_VERSION		SNDRV_PROTOCOL_VERSION(2, 0, 18)
>  
>  typedef unsigned long snd_pcm_uframes_t;
>  typedef signed long snd_pcm_sframes_t;
> @@ -330,12 +330,6 @@ enum {
>  #endif
>  };
>  
> -union snd_pcm_sync_id {
> -	unsigned char id[16];
> -	unsigned short id16[8];
> -	unsigned int id32[4];
> -};
> -
>  struct snd_pcm_info {
>  	unsigned int device;		/* RO/WR (control): device number */
>  	unsigned int subdevice;		/* RO/WR (control): subdevice number */
> @@ -348,7 +342,7 @@ struct snd_pcm_info {
>  	int dev_subclass;		/* SNDRV_PCM_SUBCLASS_* */
>  	unsigned int subdevices_count;
>  	unsigned int subdevices_avail;
> -	union snd_pcm_sync_id sync;	/* hardware synchronization ID */
> +	unsigned char pad1[16];		/* was: hardware synchronization ID */
>  	unsigned char reserved[64];	/* reserved for future... */
>  };
>  
> @@ -420,7 +414,8 @@ 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) */
> +	unsigned char reserved[48];	/* reserved for future */
>  };
>  
>  enum {

As long as I checked, the above change are safe for
backward-compatibility and compat ioctl. They keep the same size of
structure and the same offset of member in each of i386/ILP32/LP64 data
models,

> diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
> index 6f73b3c2c205..84b948db3393 100644
> --- a/sound/core/pcm_lib.c
> +++ b/sound/core/pcm_lib.c
> @@ -525,10 +525,8 @@ void snd_pcm_set_sync(struct snd_pcm_substream *substream)
>  {
>  	struct snd_pcm_runtime *runtime = substream->runtime;
>  	
> -	runtime->sync.id32[0] = substream->pcm->card->number;
> -	runtime->sync.id32[1] = -1;
> -	runtime->sync.id32[2] = -1;
> -	runtime->sync.id32[3] = -1;
> +	*(__u32 *)runtime->sync = cpu_to_le32(substream->pcm->card->number);
> +	memset(runtime->sync + 4, 0xff, sizeof(runtime->sync) - 4);
>  }
>  EXPORT_SYMBOL(snd_pcm_set_sync);

I mentioned about the issue in the above.

> @@ -1810,6 +1808,16 @@ static int snd_pcm_lib_ioctl_fifo_size(struct snd_pcm_substream *substream,
>  	return 0;
>  }
>  
> +static int snd_pcm_lib_ioctl_sync_id(struct snd_pcm_substream *substream,
> +				     void *arg)
> +{
> +	struct snd_pcm_hw_params *params = arg;
> +
> +	if (pcm_sync_empty(params->sync))
> +		memcpy(params->sync, &substream->runtime->sync, sizeof(params->sync));
> +	return 0;
> +}
> +

I mentioned about the issue in the above.

>  /**
>   * snd_pcm_lib_ioctl - a generic PCM ioctl callback
>   * @substream: the pcm substream instance
> @@ -1831,6 +1839,8 @@ int snd_pcm_lib_ioctl(struct snd_pcm_substream *substream,
>  		return snd_pcm_lib_ioctl_channel_info(substream, arg);
>  	case SNDRV_PCM_IOCTL1_FIFO_SIZE:
>  		return snd_pcm_lib_ioctl_fifo_size(substream, arg);
> +	case SNDRV_PCM_IOCTL1_SYNC_ID:
> +		return snd_pcm_lib_ioctl_sync_id(substream, arg);
>  	}
>  	return -ENXIO;
>  }
> diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> index 0b76e76823d2..63fcb08ee93d 100644
> --- a/sound/core/pcm_native.c
> +++ b/sound/core/pcm_native.c
> @@ -533,6 +533,12 @@ static int fixup_unreferenced_params(struct snd_pcm_substream *substream,
>  					  SNDRV_PCM_INFO_MMAP_VALID);
>  	}
>  
> +	err = snd_pcm_ops_ioctl(substream,
> +				SNDRV_PCM_IOCTL1_SYNC_ID,
> +				params);
> +	if (err < 0)
> +		return err;
> +
>  	return 0;
>  }
>  
> diff --git a/sound/pci/emu10k1/p16v.c b/sound/pci/emu10k1/p16v.c
> index e7f097cae574..773725dbdfbd 100644
> --- a/sound/pci/emu10k1/p16v.c
> +++ b/sound/pci/emu10k1/p16v.c
> @@ -174,10 +174,9 @@ static int snd_p16v_pcm_open_playback_channel(struct snd_pcm_substream *substrea
>  	if (err < 0)
>                  return err;
>  
> -	runtime->sync.id32[0] = substream->pcm->card->number;
> -	runtime->sync.id32[1] = 'P';
> -	runtime->sync.id32[2] = 16;
> -	runtime->sync.id32[3] = 'V';
> +	*(__u32 *)runtime->sync = cpu_to_le32(substream->pcm->card->number);
> +	memset(runtime->sync + 4, 0, sizeof(runtime->sync) - 4);
> +	strncpy(runtime->sync + 4, "P16V", 4);
  
The serialization would not keep the same data before the change, since
the third, fourth, and fifth elements are 0x31, 0x36, and 0x56, instead
of 0x10, 0x56, and 0x00.

>  	return 0;
>  }
> -- 
> 2.43.0
>