On Thu, Jun 10, 2021 at 10:08:39AM +0200, Takashi Iwai wrote:
> On Thu, 10 Jun 2021 10:05:21 +0200,
> Takashi Sakamoto wrote:
> >
> > On Thu, Jun 10, 2021 at 09:39:37AM +0200, Takashi Iwai wrote:
> > > On Thu, 10 Jun 2021 01:16:23 +0200,
> > > Takashi Sakamoto wrote:
> > > >
> > > > On Wed, Jun 09, 2021 at 05:27:29PM +0200, Takashi Iwai wrote:
> > > > > On Wed, 09 Jun 2021 16:31:43 +0200,
> > > > > Takashi Sakamoto wrote:
> > > > > > diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
> > > > > > index b7e3d8f44511..3488ec1e3674 100644
> > > > > > --- a/sound/core/pcm_lib.c
> > > > > > +++ b/sound/core/pcm_lib.c
> > > > > > @@ -1778,27 +1778,41 @@ 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.
> > > > > > + *
> > > > > > + * This 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 preferrable 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 in which lock of PCM substream is already acquired. This function may not
> > > > > > + * sleep.
> > > > >
> > > > > Hm, this text still remains here. Overlooked?
> > > >
> > > > It's my intension for documentation of
> > > > snd_pcm_period_elapsed_under_stream_lock() since it's expected to call
> > > > it under acquired lock. Its implementation doesn't yield processor
> > > > voluntarily by itself. If it yielded, it would depend on implementation
> > > > of each driver for struct snd_pcm_ops.{pointer, trigger, get_time_info},
> > > > but it's not preferable implementation of driver, in my opinion.
> > >
> > > My point is again about the sleep. This function may sleep in the
> > > nonatomic mode. The type of the PCM stream lock depends on it.
> >
> > Would I simply request you to show how the added function yields except
> > for the driver implementation? The lock of stream is expected to be
> > acquired already.
>
> In the nonatomic mode, the PCM stream lock is a mutex (no
> spin_lock_irqsave), hence it can sleep -- which contradicts with the
> added description above.
>
> Or do I misunderstand your question...?
Thanks to clarify the role of PCM stream lock, and I'm ease that we have
the same understanding about the lock.
Here, let us see deleted/added line again.
> diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
> index b7e3d8f44511..3488ec1e3674 100644
> --- a/sound/core/pcm_lib.c
> +++ b/sound/core/pcm_lib.c
> @@ -1778,27 +1778,41 @@ 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.
> + ...
> + * Context: Any context in which lock of PCM substream is already acquired. This function may not
> + * sleep.
The issued documentation is for the new function. Inner the function, the
lock of PCM substream is not acquired again since it causes dead lock
(it's not nest-able lock) regardless of usage of mutex or spin_lock.
The well-known function, snd_pcm_period_elapsed(), is rewritten to call
the new function between lock/unlock operations:
->snd_pcm_period_elapsed()
->snd_pcm_stream_lock_irqsave()
->snd_pcm_period_elapsed_under_stream_lock()
->snd_pcm_stream_unlock_irqrestore()
Or the new function can acquire the lock somewhere I overlook? However I
think it is unlikely since it necessarily causes dead lock or corruption
of irq context...
Thanks
Takashi Sakamoto
