Re: sound through enbd

"Also sprach Ming Zhang:"
> interesting. so if over wireless network, this is a media box.

mplayer seems to want to use ioctls to the oss audio device

  ioctl(7, SNDCTL_DSP_GETOSPACE, 0x859e444) = 0

  strace mplayer sample-hold.mp3 -ao oss:/dev/audio

Anyone know what that's about? If it's all it's doing, and it is, then
it must carry the data:

  ioctl(7, SNDCTL_DSP_GETOSPACE, 0x859e444) = 0
  nanosleep({0, 10000000}, NULL)          = 0
  ioctl(7, SNDCTL_DSP_GETOSPACE, 0x859e444) = 0
  nanosleep({0, 10000000}, NULL)          = 0
  ioctl(7, SNDCTL_DSP_GETOSPACE, 0x859e444) = 0

That's every 10ms. 100Hz. If that's a 44KHz, stereo 16bit stream, that
would be 4B abut 44K times a second, or 176KB/s, which would be 1.7KB
per 10ms, which is believable - but I thought ioctls were limited to
carry 8K by cross-architecture restrictions, so it can't go much faster.

Anyone want to tell me more about SNDCTL_DSP_GETOSPACE?


   #define SNDCTL_DSP_GETOSPACE            _SIOR ('P',12, audio_buf_info)

That says it's an indirected read of a type called audio_buf_info. Fine
if it contains an integrated buffer. If the buffer is external and
merely pointed to, there's trouble, but I may be able to cope.

 typedef struct audio_buf_info {
     int fragments;  /* # of available fragments (partially unsent ones not counted) */
     int fragstotal; /* Total # of fragments allocated */
     int fragsize;   /* Size of a fragment in bytes */

     int bytes;      /* Available space in bytes (includes partially used fragments) */
                     /* Note! 'bytes' could be more than fragments*fragsize */
 } audio_buf_info;

Wellll .. for sure there's no internal buffer. The good news is there's
no pointer there. If the news is really good, the data will follow the
ioctl struct and be ... fragments*fragsize long. Maybe "bytes" long.

Arrrrgh. I think they are aiming for using dma! Dma over the net is not
going to flow ...

                case SNDCTL_DSP_GETOSPACE:
                        if (!(audio_devs[dev]->open_mode & OPEN_WRITE))
                                return -EPERM;
                        if ((audio_devs[dev]->audio_mode & AM_READ) && !(audio_d
evs[dev]->flags & DMA_DUPLEX))
                                return -EBUSY;
                        return dma_ioctl(dev, cmd, arg);
                               ^^^^

Curses! Anyone have any insight? Or even knowledge? About audio? Is
there some transfer mode that doesn't use dma? Can I trick the calling
application (mplayer, here) into using it?

I would guess so - at some point there msut have been a question sent
out about capabilities.

Audio experts?

Peter