[alsa-devel] [PATCH 1/1] pcm:file: delegate htimestamping to slave instead of always getting real_htimestamp

[Takashi Iwai]

On Thu, 23 Mar 2017 12:40:45 +0100,
sutar.mounesh at gmail.com wrote:
> 
> From: Andreas Pape <apape at de.adit-jv.com>
> 
> purpose of this fix, is to read most accurate timestamps.
> 
>  From documentation of /src/pcm/pcm.c, we can see:
>  """" \par Timestamp mode
> 
>  The timestamp mode specifies, if timestamps are activated. Currently, only #SND_PCM_TSTAMP_NONE and #SND_PCM_TSTAMP_MMAP modes are known.
>  The mmap mode means that timestamp is taken on every period time boundary. Corresponding position in the ring buffer assigned to timestamp can be obtained using #snd_pcm_htimestamp() function. """"
> 
>  As snd_pcm_generic_htimestamp() internally calls snd_pcm_htimestamp() to read time, so accurate timestamp can be read from snd_pcm_generic_htimestamp().
> 
>  Also, in case of pcm_file, if the underlying slave is hardware, then we would wish to read elapsed hardware time, as it will be the most accurate, as opposed to the elapsed wall time.
>  This will provide pcm_file with the most accurate timestamps.
> 
>  Following are the timesamps read with timestamp enabled, for with fix and without fix scenarios:
> 
>  1> With fix:
> :~#time aplay --enable-tstamp -Dhtstamp_test --period-time=5000 -v -fdat /dev/urandom
> Playing raw data '/dev/urandom' : Signed 16 bit Little Endian, Rate 48000 Hz, Stereo
> File PCM (file=/tmp/swarate_out.wav)
> Final file PCM (file=/tmp/swarate_out.wav)
> ..
> 
> Slave: Hardware PCM card 0 'imx6q-sabresd-wm8962' device 0 subdevice 0
> Its setup is:
>   stream       : PLAYBACK
>   access       : RW_INTERLEAVED
>   format       : S16_LE
>   subformat    : STD
>   channels     : 2
>   rate         : 48000
>   exact rate   : 48000 (48000/1)
>   msbits       : 16
>   buffer_size  : 24000
>   period_size  : 240
>   period_time  : 5000
>   tstamp_mode  : ENABLE
>   .
>   .
> Before sleep = 142:409.807623
> After sleep = 142:409.807623
> Before sleep = 142:414.806016 (calling snd_pcm_htimestamp(handle, &avail, &tstamp_before))
> sleep of 2 milisec
> After sleep = 142:414.806016 (calling snd_pcm_htimestamp(handle, &avail, &tstamp_after)
> >From the above timestamps, we can see that slave has returned the same timestamps, as --period-time choosen is 5msec.
> 
> 2> Without this fix:
> The timestamps are returned with realtime value.
> 
> :~# time aplay --enable-tstamp -Dhtstamp_test --period-time=5000 -v -fdat /dev/urandom
> Playing raw data '/dev/urandom' : Signed 16 bit Little Endian, Rate 48000 Hz, Stereo
> File PCM (file=/tmp/swarate_out.wav)
> Final file PCM (file=/tmp/swarate_out.wav)
> .
> .
> Slave: Hardware PCM card 0 'imx6q-sabresd-wm8962' device 0 subdevice 0
> Its setup is:
>   stream       : PLAYBACK
>   access       : RW_INTERLEAVED
>   format       : S16_LE
>   subformat    : STD
>   channels     : 2
>   rate         : 48000
>   exact rate   : 48000 (48000/1)
>   msbits       : 16
>   buffer_size  : 24000
>   period_size  : 240
>   period_time  : 5000
>   tstamp_mode  : ENABLE
> .
> .
> Before sleep = 241:136.875845 (calling snd_pcm_htimestamp(handle, &avail, &tstamp_before))
> sleep of 2 milisec
> After sleep = 241:139.076376 (calling snd_pcm_htimestamp(handle, &avail, &tstamp_after)
> 
> We can observe here, the timestamps shows time diff of ~2ms, which is the time gap of sleep duration.
> Before sleep = 241:139.617588
> After sleep = 241:141.746845
> Before sleep = 241:142.291618
> After sleep = 241:144.406406
> Before sleep = 241:144.951421
> After sleep = 241:147.066118
> Before sleep = 241:147.623421
> After sleep = 241:149.740573
> 
> Signed-off-by: Andreas Pape <apape at de.adit-jv.com>
> Signed-off-by: Mounesh Sutar <sutar.mounesh at gmail.com>

Applied now, thanks.


Takashi