[alsa-devel] [RFC - AAF PCM plugin 5/5] aaf: Implement Capture mode support

[Takashi Sakamoto]

Hi,

On Aug 21 2018 10:06, Andre Guedes wrote:
> This patch implements the capture mode support from the AAF plugin.
> Simply put, this mode works as follows: AVTP packets are received from
> the network, the PCM samples are retrieved and presented to the alsa-lib
> layer at the presentation time. In summary, the capture mode implements
> a typical AVTP Listener.
> 
> Once the AAF device is put in running state, packet reception is
> started. Every time an AVTP packet is received the plugin checks if it
> is valid (according to the stream configuration provided by the user)
> and copies the PCM samples to the audio buffer. Note that at this
> moment, the samples are not presented to the alsa-lib layer (i.e.
> hw_ptr is not updated).
> 
> When the first AVTP packet is received, the media clock is set to start
> at the presentation time from that packet. At every tick from the media
> clock, PCM samples are presented to the alsa-lib layer.
> 
> Signed-off-by: Andre Guedes <andre.guedes at intel.com>
> ---
>   aaf/pcm_aaf.c | 358 ++++++++++++++++++++++++++++++++++++++++++++++++++++++----
>   1 file changed, 338 insertions(+), 20 deletions(-)
> 
> diff --git a/aaf/pcm_aaf.c b/aaf/pcm_aaf.c
> index e098247..b5bee57 100644
> --- a/aaf/pcm_aaf.c
> +++ b/aaf/pcm_aaf.c
> ...
> @@ -561,6 +819,42 @@ static int aaf_mclk_timeout_playback(snd_pcm_aaf_t *aaf)
>   	return 0;
>   }
>   
> +static int aaf_mclk_timeout_capture(snd_pcm_aaf_t *aaf)
> +{
> +	ssize_t n;
> +	uint64_t expirations;
> +
> +	n = read(aaf->timer_fd, &expirations, sizeof(uint64_t));
> +	if (n < 0) {
> +		SNDERR("Failed to read() timer");
> +		return -errno;
> +	}
> +
> +	if (expirations != 1)
> +		pr_debug("Missed %llu presentation time(s) ", expirations - 1);
> +
> +	while (expirations--) {
> +		snd_pcm_sframes_t len;
> +
> +		aaf_inc_ptr(&aaf->hw_ptr, aaf->frames_per_pkt, aaf->boundary);
> +
> +		len = aaf->hw_virt_ptr - aaf->hw_ptr;
> +		if (len < 0)
> +			len += aaf->boundary;
> +		if (len > aaf->buffer_size) {
> +			/* If the distance between hw virtual pointer and hw
> +			 * pointer is greater than the buffer size, it means we
> +			 * had an overrun error so -EPIPE is returned.
> +			 */
> +			return -EPIPE;
> +		}
> +
> +		aaf->mclk_ticks++;
> +	}
> +
> +	return 0;
> +}

In your code, -EPIPE can be returned in a call of 
'snd_pcm_poll_revents()'. In this case, typical applications follow 
recovery process below:

->snd_pcm_poll_revents()
->snd_pcm_prepare()
->snd_pcm_start()

In this case, status of corresponding PCM substream is reset (see 
'snd_pcm_ioplug_prepare()'). In my opinion, in this case, backend is 
expected to stop data streaming, then initialize hardware and restart 
data streaming. In your driver, timer_fd/sk_fd is leaked. It's better to
release them by checking status of PCM substream in .prepare callback.


Regards

Takashi Sakamoto