[alsa-devel] period_size and relation to number of samples

[Takashi Iwai]

At Thu, 06 Sep 2007 07:52:37 +0200,
Markus Korber wrote:
> 
> Hello,
> 
> one question: If my driver announces the setting below[1], how many
> samples must an application provide in a single snd_pcm_writei() or
> snd_pcm_mmap_writei() call?
> 
> Shouldn't it always send 8192 bytes or 2048 left and right samples (4096
> in total) or is it free to send more or less?

The transfer size is aligned to xfer_align, but besides that, it's
free for apps.

> Logging data in my chip_playback_copy() function shows the following
> (S16_LE, RW_INTERLEAVED):
> 
> ,----[ mplayer, aplay: ]
> | chip_playback_copy (0 -> src: 0xffce1000, #bytes: 0x4000)
> | chip_playback_copy (1 -> src: 0xffce1000, #bytes: 0x2000)
> | chip_playback_copy (1 -> src: 0xffce1000, #bytes: 0x2000)
> `----
> 
> ,----[ alsaplayer: ]
> | chip_playback_copy (0 -> src: 0xffce1000, #bytes: 0x2000)
> | chip_playback_copy (0 -> src: 0xffce3000, #bytes: 0x2000)
> | chip_playback_copy (0 -> src: 0xffce3000, #bytes: 0x2000)
> | [...]
> `----
> 
> Thus mplayer and aplay provide 16384 bytes (4096 l/r samples) in the
> first call and 8192 bytes (2048 l/r samples) for all subsequent calls.
> However, alsaplayer provides 8192 bytes (2048 l/r samples) for all
> calls.
> 
> Now, what is an application allowed to send and what not?  For example,
> could an application only send 1024 l/r samples and is the driver
> responsible for buffering the data?  Or must it obey the announced
> period_size and *always* provide 2048 l/r samples?

No, as mentioned, the app is free to send any size in general.  When
the period size is filled up, basically it's supposed to be playable.
But, the procedure "fill the whole buffer then start" is the most
robust way.  

The period size is the minimal chunk size that controls the poll
frequency.  So, it's natural to send in this size.  It's no
requirement but a common use case.


Takashi

> Thanks.
> 
> ,----[ 1 ]
> | stream       : PLAYBACK
> | access       : RW_INTERLEAVED
> | format       : S16_LE
> | subformat    : STD
> | channels     : 2
> | rate         : 44100
> | exact rate   : 44100 (44100/1)
> | msbits       : 16
> | buffer_size  : 4096
> | period_size  : 2048
> | period_time  : 46439
> | tick_time    : 10000
> | tstamp_mode  : NONE
> | period_step  : 1
> | sleep_min    : 0
> | avail_min    : 2048
> | xfer_align   : 1
> | start_threshold  : 2048
> | stop_threshold   : 4096
> | silence_threshold: 0
> | silence_size : 0
> | boundary     : 1073741824
> | start_mode: DATA
> | xrun_mode: STOP
> | tstamp_mode: NONE
> | period_step: 1
> | sleep_min: 0
> | avail_min: 2048
> | xfer_align: 1
> | silence_threshold: 0
> | silence_size: 0
> | boundary: 1073741824
> `----
> 
> Regards,
> Markus Korber
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