[alsa-devel] How to enforce 32bit access?

[Takashi Iwai]

At Wed, 10 Oct 2007 17:40:08 +0200,
Joachim Foerster wrote:
> 
> On Wed, 2007-10-10 at 15:22 +0200, Takashi Iwai wrote:
> > > > At Fri, 05 Oct 2007 19:09:53 +0200,
> > > > Joachim Foerster wrote:
> > > > > 
> > > > > Hi ALSA devs,
> > > > > 
> > > > > meanwhile I tried to put a constraint on SNDRV_PCM_HW_PARAM_FRAME_BITS
> > > > > by the use of snd_pcm_hw_constraint_minmax() function with min=32 and
> > > > > max=32, but still no success - the ALSA Library still makes 16bit
> > > > > accesses, when playing mono (1 channel) files.
> > > > > 
> > > > > Any ideas?
> > > > 
> > > > Could you show the chain of plugins via aplay -v ?
> > > > I'm not sure which plugin requires S16.  Possibly, the rate converter.
> > > 
> > > bash-3.00# aplay -v -M ../Absage.wav
> > > Playing WAVE '../Absage.wav' : Signed 16 bit Little Endian, Rate 16000
> > > Hz, Mono
> > 
> > Well, the sample itself is 16bit format, and your hardware *does*
> > support 16bit format.  Why to be 32bit?
> 
> Well actually, we are on a embedded system here. The bus is 32bit wide
> (Xilinx ML403, PowerPC 405, OBP). So, why waste resources with two 16bit
> accesses?

Because you programmed in such a way :)

> One sample is 16bits. But we have two channels (stereo only). Thus the
> controller expects a frame with 32bit data - two samples at the same
> time.
> 
>  Joachim
> 
> PS: As soon as stereo .wav-files are used, there is no problem, because
> then the "conversion framework" and the Plug PCM are not used - just a
> plain memcpy which results in 32bit accesses.

The problem is that the driver still accepts the 16bit format mono
stream.  That's what I pointed in my previous post.  There is no
format conversion in alsa-lib at all.  It's only channel expansion
from mono to stereo.  The hardware accepts 16bit stereo.  So, there is
nothing wrong from this viewpoint.

Maybe it's not about plugin but a problem of mmap with your device?


Plug PCM: Route conversion PCM (sformat=S16_LE)
  Transformation table:
    0 <- 0
    1 <- 0
Its setup is:
  stream       : PLAYBACK
  access       : MMAP_INTERLEAVED
  format       : S16_LE
  subformat    : STD
  channels     : 1
  rate         : 16000
  exact rate   : 16000 (16000/1)
  msbits       : 16
  buffer_size  : 4096
  period_size  : 1024
  period_time  : 64000
  tick_time    : 4000
  tstamp_mode  : NONE
  period_step  : 1
  sleep_min    : 0
  avail_min    : 1024
  xfer_align   : 1024
  start_threshold  : 4096
  stop_threshold   : 4096
  silence_threshold: 0
  silence_size : 0
  boundary     : 1073741824
Slave: Hardware PCM card 0 'Lorenz' AC97 Digital Controller' device 0
subdevice 0
Its setup is:
  stream       : PLAYBACK
  access       : MMAP_INTERLEAVED
  format       : S16_LE
  subformat    : STD
  channels     : 2
  rate         : 16000
  exact rate   : 16000 (16000/1)
  msbits       : 16
  buffer_size  : 4096
  period_size  : 1024
  period_time  : 64000
  tick_time    : 4000
  tstamp_mode  : NONE
  period_step  : 1
  sleep_min    : 0
  avail_min    : 1024
  xfer_align   : 1024
  start_threshold  : 4096
  stop_threshold   : 4096
  silence_threshold: 0
  silence_size : 0
  boundary     : 1073741824


Takashi