Re: [PATCH 2/2] ALSA: pcm: optimize and clarify stream sychronization ID API

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

On Mon, May 06, 2024 at 05:11:50PM +0200, Jaroslav Kysela wrote:
> Optimize the memory usage in struct snd_pcm_runtime - use boolean
> value for the standard sync ID scheme.
> 
> Introduce snd_pcm_set_sync_per_card function to build synchronization
> IDs.
> 
> Signed-off-by: Jaroslav Kysela <perex@perex.cz>
> ---
>  include/sound/pcm.h      |  9 +++++++--
>  sound/core/pcm_lib.c     | 28 +++++++++++++++++-----------
>  sound/pci/emu10k1/p16v.c | 16 ++++++++++++----
>  3 files changed, 36 insertions(+), 17 deletions(-)
> 
> diff --git a/include/sound/pcm.h b/include/sound/pcm.h
> index dae41b100517..71c80ad8d4d3 100644
> --- a/include/sound/pcm.h
> +++ b/include/sound/pcm.h
> @@ -397,7 +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 */
>  
> -	unsigned char sync[16];		/* hardware synchronization ID */
> +	bool sync_flag;			/* hardware synchronization - standard per card ID */
>  
>  	/* -- mmap -- */
>  	struct snd_pcm_mmap_status *status;
> @@ -1151,7 +1151,12 @@ int snd_pcm_format_set_silence(snd_pcm_format_t format, void *buf, unsigned int
>  
>  void snd_pcm_set_ops(struct snd_pcm * pcm, int direction,
>  		     const struct snd_pcm_ops *ops);
> -void snd_pcm_set_sync(struct snd_pcm_substream *substream);
> +void snd_pcm_set_sync_per_card(struct snd_pcm_substream *substream, struct snd_pcm_hw_params *params,
> +			       const unsigned char *id, unsigned int len);
> +static inline void snd_pcm_set_sync(struct snd_pcm_substream *substream)
> +{
> +	substream->runtime->sync_flag = true;
> +}
>  int snd_pcm_lib_ioctl(struct snd_pcm_substream *substream,
>  		      unsigned int cmd, void *arg);                      
>  void snd_pcm_period_elapsed_under_stream_lock(struct snd_pcm_substream *substream);
> diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
> index 84b948db3393..82452c2e4887 100644
> --- a/sound/core/pcm_lib.c
> +++ b/sound/core/pcm_lib.c
> @@ -516,19 +516,23 @@ void snd_pcm_set_ops(struct snd_pcm *pcm, int direction,
>  EXPORT_SYMBOL(snd_pcm_set_ops);
>  
>  /**
> - * snd_pcm_set_sync - set the PCM sync id
> + * snd_pcm_set_sync_per_card - set the PCM sync id with card number
>   * @substream: the pcm substream
> + * @params: modified hardware parameters
> + * @id: identifier (max 12 bytes)
> + * @len: identifier length (max 12 bytes)
>   *
> - * Sets the PCM sync identifier for the card.
> + * Sets the PCM sync identifier for the card with zero padding.
>   */
> -void snd_pcm_set_sync(struct snd_pcm_substream *substream)
> +void snd_pcm_set_sync_per_card(struct snd_pcm_substream *substream,
> +			       struct snd_pcm_hw_params *params,
> +			       const unsigned char *id, unsigned int len)
>  {
> -	struct snd_pcm_runtime *runtime = substream->runtime;
> -	
> -	*(__u32 *)runtime->sync = cpu_to_le32(substream->pcm->card->number);
> -	memset(runtime->sync + 4, 0xff, sizeof(runtime->sync) - 4);
> +	*(__u32 *)params->sync = cpu_to_le32(substream->pcm->card->number);
> +	len = max(12, len);
> +	strncpy(params->sync + 4, id, len);
> +	memset(params->sync + 4 + len, 0, 12 - len);
>  }
> -EXPORT_SYMBOL(snd_pcm_set_sync);
  
As I mentioned to the previous patch, the value of zero in the first
element of array is problematic, since it is not distinguishable from
the card indexed by zero.

Additionally, I'm worried the second, third, and fourth element of array
is kept as zero, against 0xff in the original serialization code.

I think the series of '0x00 0x00 0x00 numeric-card-id' in the first four
elements is more preferable, however I wonder why the data is typed as
character array instead of integer array.

If the intension of API is not the kind of 'forcing-policy', the byte data
passed from drivers to user space application is reasonable. However,
in the call of above kernel API results in the breakage of policy. I think
more sophisticated way should be investigated, instead of the change in
this patch, in a view of programming interface for device driver developers
and user space application developers.

>  /*
>   *  Standard ioctl routine
> @@ -1811,10 +1815,12 @@ static int snd_pcm_lib_ioctl_fifo_size(struct snd_pcm_substream *substream,
>  static int snd_pcm_lib_ioctl_sync_id(struct snd_pcm_substream *substream,
>  				     void *arg)
>  {
> -	struct snd_pcm_hw_params *params = arg;
> +	static unsigned char id[12] = { 0xff, 0xff, 0xff, 0xff,
> +					0xff, 0xff, 0xff, 0xff,
> +					0xff, 0xff, 0xff, 0xff };
>  
> -	if (pcm_sync_empty(params->sync))
> -		memcpy(params->sync, &substream->runtime->sync, sizeof(params->sync));
> +	if (substream->runtime->sync_flag)
> +		snd_pcm_set_sync_per_card(substream, arg, id, sizeof(id));
>  	return 0;
>  }
>  
> diff --git a/sound/pci/emu10k1/p16v.c b/sound/pci/emu10k1/p16v.c
> index 773725dbdfbd..36b8a4ce2081 100644
> --- a/sound/pci/emu10k1/p16v.c
> +++ b/sound/pci/emu10k1/p16v.c
> @@ -174,10 +174,6 @@ static int snd_p16v_pcm_open_playback_channel(struct snd_pcm_substream *substrea
>  	if (err < 0)
>                  return err;
>  
> -	*(__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);
> -
>  	return 0;
>  }
>  
> @@ -225,6 +221,17 @@ static int snd_p16v_pcm_open_capture(struct snd_pcm_substream *substream)
>  	return snd_p16v_pcm_open_capture_channel(substream, 0);
>  }
>  
> +static int snd_p16v_pcm_ioctl_playback(struct snd_pcm_substream *substream,
> +				       unsigned int cmd, void *arg)
> +{
> +	if (cmd == SNDRV_PCM_IOCTL1_SYNC_ID) {
> +		static unsigned char id[4] = { 'P', '1', '6', 'V' };
> +		snd_pcm_set_sync_per_card(substream, arg, id, 4);
> +		return 0;
> +	}
> +	return snd_pcm_lib_ioctl(substream, cmd, arg);
> +}
> +

'static const unsigned char id[4]' is preferable, since the data is
immutable during the lifetime of kernel module.

Anyway, as I mentioned to the previous patch, the serialization is not
compatible.

>  /* prepare playback callback */
>  static int snd_p16v_pcm_prepare_playback(struct snd_pcm_substream *substream)
>  {
> @@ -530,6 +537,7 @@ snd_p16v_pcm_pointer_capture(struct snd_pcm_substream *substream)
>  static const struct snd_pcm_ops snd_p16v_playback_front_ops = {
>  	.open =        snd_p16v_pcm_open_playback_front,
>  	.close =       snd_p16v_pcm_close_playback,
> +	.ioctl =       snd_p16v_pcm_ioctl_playback,
>  	.prepare =     snd_p16v_pcm_prepare_playback,
>  	.trigger =     snd_p16v_pcm_trigger_playback,
>  	.pointer =     snd_p16v_pcm_pointer_playback,
> -- 
> 2.43.0