[alsa-devel] Stream format conversion in SoC driver

[Giedrius Trainavičius]

Hey,

Any thoughts or pointers on what to look for in order to use different
sample format between actual I2S and DMA?

Thank you,
Giedrius.

On Thu, Dec 29, 2016 at 4:32 PM, Giedrius Trainavičius <
giedrius.trainavicius at gmail.com> wrote:

> The I2S and DMA format asymmetry sounds like what I'd be after. Do you
> have any pointers for APIs / keywords to look around for in the code to see
> how this should be set up? Is such format conversion supposed to be
> happening in hardware itself, or the CPU when accessing the DMA buffer has
> the chance to do the conversion?
>
> The relevant documentation and code I found so far would be:
>
> https://www.raspberrypi.org/documentation/hardware/raspberry
> pi/bcm2835/BCM2835-ARM-Peripherals.pdf
> https://github.com/raspberrypi/linux/blob/rpi-4.4.y/sound/
> soc/bcm/bcm2835-i2s.c
>
> I can't locate any relevant bits of code or information that'd indicate
> capabilities of sample format conversion...
>
> Thank you,
> Giedrius.
>
> On Thu, Dec 29, 2016 at 7:53 AM, Clemens Ladisch <clemens at ladisch.de>
> wrote:
>
>> Giedrius Trainavičius wrote:
>> >It uses I2S for sending / receiving audio. However, the master clock on
>> our
>> >sound card is generated by the ADC, which supports only 24 bit data, so
>> >getting other sample formats is not possible.
>>
>> The I2S sample format need not be the same as what the DMA
>> controller reads/writes from/to memory.
>>
>> >A question on 'plug' - it accepts 'nearest' as the argument for format,
>> >rate, what happens in case the slave pcm stream supports the exact same
>> >format as requested by user software?
>>
>> The plug plugin instantiates the actual conversion plugins only
>> as needed; with no conversion, it vanishes entirely.
>>
>>
>> Regards,
>> Clemens
>>
>>
>