pcm:file: delegate htimestamping to slave instead of always getting real_htimestamp

pcm:file: delegate htimestamping to slave instead of always getting real_htimestamp

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@de.adit-jv.com>
Signed-off-by: Mounesh Sutar <sutar.mounesh@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>