[alsa-devel] [RFC - AAF PCM plugin 3/5] aaf: Implement Playback mode support
Pierre-Louis Bossart
pierre-louis.bossart at linux.intel.com
Wed Aug 22 00:51:04 CEST 2018
>>> +static int aaf_mclk_start_playback(snd_pcm_aaf_t *aaf)
>>> +{
>>> + int res;
>>> + struct timespec now;
>>> + struct itimerspec itspec;
>>> + snd_pcm_ioplug_t *io = &aaf->io;
>>> +
>>> + res = clock_gettime(CLOCK_REF, &now);
>>> + if (res < 0) {
>>> + SNDERR("Failed to get time from clock");
>>> + return -errno;
>>> + }
>>> +
>>> + aaf->mclk_period = (NSEC_PER_SEC * aaf->frames_per_pkt) /
>>> io->rate;
>>
>> is this always an integer? If not, don't you have a systematic
>> arithmetic error?
>
> NSEC_PER_SEC is 64-bit so I don't see an arithmetic error during
> calculation (e.g. integer overflow). Not sure this was your concern,
> though. Let me know otherwise.
No, I was talking about the fractional part, e.g with 256 frames with
44.1kHz you have a period of 5804988.662131519274376 - so your math adds
a truncation. same with 48khz, the fractional part is .333
I burned a number of my remaining neurons chasing a <100 ppb error which
led to underruns after 10 hours, so careful now with truncation...
>
>>> +static int aaf_poll_revents(snd_pcm_ioplug_t *io, struct pollfd
>>> *pfd,
>>> + unsigned int nfds, unsigned short
>>> *revents)
>>> +{
>>> + int res;
>>> + snd_pcm_aaf_t *aaf = io->private_data;
>>> +
>>> + if (nfds != FD_COUNT_PLAYBACK)
>>> + return -EINVAL;
>>> +
>>> + if (pfd[0].revents & POLLIN) {
>>> + res = aaf_mclk_timeout_playback(aaf);
>>> + if (res < 0)
>>> + return res;
>>> +
>>> + *revents = POLLIN;
>>> + }
>>
>> I couldn't figure out how you use playback events and your timer.
>
> Every time aaf->timer_fd expires, the audio buffer is consumed by the
> plugin, making some room available on the buffer. So here a POLLIN
> event is returned so alsa-lib layer can copy more data into the audio
> buffer.
>
>> When there are two audio clock sources or timers that's usually where
>> the fun begins.
>
> Regarding scenarios with two audio clock sources or timers, the plugin
> doesn't support them at the moment. This is something we should work on
> once the basic functionality is pushed upstream.
I was talking about adjusting the relationship between your
CLOCK_REALTIME timer and the media/network clock. I don't quite get how
this happens, I vaguely recall there should be a daemon which tracks the
difference between local and media/network clock, and I don't see it here.
>
> Thank you for your feedback.
>
> Regards,
>
> Andre
>
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