Thanks Clemens
>> is the best method to build a sound card and a capture PCM?
>As compared to what? Any other method will not allow your hardware to
be used for audio recording.
As compared to my driver configuring the DMA to capture the data into a
known buffer, and then directly accessing the buffer to wrap it in a WAV
format. I should have rephrased my initial question as, "my goal is to
simply wrap multichannel digital mic input in WAV format; is ALSA/arecord
the most efficient way to do this?"
>> Most of the tutorials seem to imply an IRQ signals a frame of data has
>> transferred
>No, the DMA interrupt typically signals that a period of data has been
transferred.
OK, so per the alsa wiki
<http://www.alsa-project.org/main/index.php/FramesPeriods>, I should make
my buffer at least 2x the size of a period. I could in theory accept
several frames of data based on my desired period size, fire the IRQ, and
then....? I have a buffer ready, under which function does it get passed
back to the ALSA framework? Ben Collins uses copy_to_user, but I'm keeping
this all in kernel space (or is that wrong?).
The flow I think I need, please correct me if I'm wrong or missing. I can
dump code if it's more helpful:
- Create Card
- Create Device
- Create PCM, 2 channel
- Define hardware, ops, etc.
- preallocate_pages
- register card.
- (arecord is called)
- pcm_trigger_start called
- dma begins transfer (of size #periods in a circular buffer)
- dma completes/fires period-based IRQ
- <<question>> buffer is transferred in function X (?) to Y (?)
- ALSA internally checks if # of periods has been transferred (based on
IRQ count somewhere?)
- pcm_trigger_stop is called, stopping dma
- arecord returns w/ file saved to filesystem
>How exactly does your DMA controller work?
I have a 2 channel digital mic, running at 24bit, 44.1KHz. I've written
FPGA code to buffer it, and write the data into a buffer in DRAM that the
processor has provided. I can configure the number of frames/periods
transferred before I fire an IRQ. It uses a circular buffer descriptor ring.
>(And development questions belong on the alsa-devel list.)
I just requested access to that list, but haven't received approval yet.
Thanks again for all the help!
Rob
On Tue, May 3, 2016 at 1:50 AM, Clemens Ladisch <cladi...@googlemail.com>
wrote:
> Rob Nertney wrote:
> > is the best method to build a sound card and a capture PCM?
>
> As compared to what? Any other method will not allow your hardware to
> be used for audio recording.
>
> > do I copy the DMA buffer address in the TRIGGER case statements?
>
> Your trigger callback is responsible for starting and stopping the DMA;
> nothing else.
>
> > I'm not sure where to do the framing of the data.
>
> This is done by the hardware. Your driver just describes the format of
> the data.
>
> > Most of the tutorials seem to imply an IRQ signals a frame of data has
> > transferred
>
> No, the DMA interrupt typically signals that a period of data has been
> transferred.
>
> How exactly does your DMA controller work?
>
>
> (And development questions belong on the alsa-devel list.)
>
>
> Regards,
> Clemens
>
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