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 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, 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.)
>(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 <cladisch@xxxxxxxxxxxxxx> 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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