Re: [PATCH 1/3] ALSA: pcm: add snd_pcm_period_elapsed() variant without acquiring lock of PCM substream

[Takashi Iwai <tiwai@suse.de>]

On Wed, 09 Jun 2021 03:22:42 +0200,
Takashi Sakamoto wrote:
> 
> Current implementation of ALSA PCM core has a kernel API,
> snd_pcm_period_elapsed(), for drivers to awaken processes from waiting for
> available frames. The function voluntarily acquires lock of PCM substream,
> therefore it is not called in process context for any PCM operation since
> the lock is already acquired.
> 
> The call in process context is convenient for packet-oriented driver, at
> least for drivers to audio and music unit in IEEE 1394 bus. The drivers
> are allowed by Linux FireWire subsystem to process isochronous packets
> queued till recent isochronous cycle in process context in any time.
> 
> This commit adds snd_pcm_period_elapsed() variant,
> snd_pcm_period_elapsed_under_stream_lock(), for drivers to queue the event
> in the process context.
> 
> Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
> ---
>  include/sound/pcm.h  |  1 +
>  sound/core/pcm_lib.c | 68 ++++++++++++++++++++++++++++++++++----------
>  2 files changed, 54 insertions(+), 15 deletions(-)
> 
> diff --git a/include/sound/pcm.h b/include/sound/pcm.h
> index 2e1200d17d0c..bae90696cd06 100644
> --- a/include/sound/pcm.h
> +++ b/include/sound/pcm.h
> @@ -1066,6 +1066,7 @@ void snd_pcm_set_ops(struct snd_pcm * pcm, int direction,
>  void snd_pcm_set_sync(struct snd_pcm_substream *substream);
>  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);
>  void snd_pcm_period_elapsed(struct snd_pcm_substream *substream);
>  snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream,
>  				     void *buf, bool interleaved,
> diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
> index b7e3d8f44511..6f01b0c805ca 100644
> --- a/sound/core/pcm_lib.c
> +++ b/sound/core/pcm_lib.c
> @@ -1778,27 +1778,40 @@ int snd_pcm_lib_ioctl(struct snd_pcm_substream *substream,
>  EXPORT_SYMBOL(snd_pcm_lib_ioctl);
>  
>  /**
> - * snd_pcm_period_elapsed - update the pcm status for the next period
> - * @substream: the pcm substream instance
> + * snd_pcm_period_elapsed_under_stream_lock() - update the status of runtime for the next period
> + *						under acquired lock of PCM substream.
> + * @substream: the instance of pcm substream.
> + *
> + * The function is called when the batch of audio data frames as the same size as the period of
> + * buffer is already processed in audio data transmission.
> + *
> + * The call of function updates the status of runtime with the latest position of audio data
> + * transmission, checks overrun and underrun over buffer, awaken user processes from waiting for
> + * available audio data frames, sampling audio timestamp, and performs stop or drain the PCM
> + * substream according to configured threshold.
> + *
> + * The function is intended to use for the case that PCM driver operates audio data frames under
> + * acquired lock of PCM substream; e.g. in callback of any operation of &snd_pcm_ops in process
> + * context. In any interrupt context, it's preferable to use ``snd_pcm_period_elapsed()`` instead
> + * since lock of PCM substream should be acquired in advance.
>   *
> - * This function is called from the interrupt handler when the
> - * PCM has processed the period size.  It will update the current
> - * pointer, wake up sleepers, etc.
> + * Developer should pay enough attention that some callbacks in &snd_pcm_ops are done by the call of
> + * function:
>   *
> - * Even if more than one periods have elapsed since the last call, you
> - * have to call this only once.
> + * - .pointer - to retrieve current position of audio data transmission by frame count or XRUN state.
> + * - .trigger - with SNDRV_PCM_TRIGGER_STOP at XRUN or DRAINING state.
> + * - .get_time_info - to retrieve audio time stamp if needed.
> + *
> + * Even if more than one periods have elapsed since the last call, you have to call this only once.
> + *
> + * Context: Any context under acquired lock of PCM substream. This function may not sleep.

Actually it may sleep if the PCM is nonatomic mode; then the stream
lock is a mutex instead of a spinlock.


Takashi