Greetings,
I'm setting out to write an ALSA driver for a very limited device thats behind
a PLD. The actual codec is a uda1380 and does 16bit MSB stereo encode/decode.
The way the device is driven the sampling rate is limited to 8KHz and data
transfer to/from the device I have to handle manually.
I've read over the 'writing an ALSA driver' tutorial and have started the
driver but have run into a few questions:
- for the specs I've given above: 8000Hz sampling rate, 16bit stereo MSB
samples does the following snd_pcm_hardware_t look right?:
static snd_pcm_hardware_t my_playback_hw = {
.info = (SNDRV_PCM_INFO_MMAP |
SNDRV_PCM_INFO_INTERLEAVED |
SNDRV_PCM_INFO_BLOCK_TRANSFER |
SNDRV_PCM_INFO_MMAP_VALID),
.formats = SNDRV_PCM_FMTBIT_U16_BE, // is this 16bit MSB?
.rates = SNDRV_PCM_RATE_8000, // seems redundant?
.rate_min = 8000,
.rate_max = 8000,
.channels_min = 2,
.channels_max = 2,
.buffer_bytes_max = 32768,
.period_bytes_min = 4096,
.period_bytes_max = 32768,
.periods_min = 1,
.periods_max = 1024,
};
- I'm a little confused at how to deal with copying data from/to the device.
It looks like I would implement the copy callback and copy the passed buffer to
the device in the device specific manner I need to (playback), but I'm not
clear where to put data that I receive from the device (capture). I notice
that when the hw_params callback is called a dma buffer has been created by
ALSA in the substream->runtime struct. I assume this is created based on hte
buffer_bytes_max field above and I'm to copy data to this buffer? How do I
know where to copy it within the buffer and what data ALSA has already consumed
from the buffer?
- I might need to create an intermediate buffer between ALSA's rather large
buffers and my hardware buffer. Is there any driver that I could look at that
does this? The tutorial talks about the vxpocket driver but that code looks
very foriegn from the rest of the ALSA drivers.
- I'm not sure I understand what the 'period' is about. Its clear to me that
I need to call snd_pcm_period_elapsed() periodically from either an IRQ or
other timed callback, but I'm not clear what the hardware pointer is or how
that converts to 'frames' and 'periods'.
- in general, if my hardware device only allows 16bit MSB 8KHz, does ALSA
provides a mechanism for converting data from other formats?
Thanks for any pointers,
Tim
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