[alsa-devel] dsp after dmix

[Michael Trimarchi]

Hi

On Mon, Oct 14, 2013 at 2:27 PM, Takashi Iwai <tiwai at suse.de> wrote:
> At Mon, 14 Oct 2013 14:18:27 +0200,
> Michael Trimarchi wrote:
>>
>> Hi
>>
>> On Mon, Oct 14, 2013 at 2:15 PM, Clemens Ladisch <clemens at ladisch.de> wrote:
>> > Michael Trimarchi wrote:
>> >> On Mon, Oct 14, 2013 at 9:08 AM, Clemens Ladisch <clemens at ladisch.de> wrote:
>> >>> Michael Trimarchi wrote:
>> >>>> My problem is to apply filtering after dmix plugin.
>> >>>
>> >>> The dmix plugin works only on top of a "hw" device.
>> >>
>> >> Yes I have seen the code. Is there anyway to work with dps without passing
>> >> thought pulseaudio?
>> >
>> > You would have to write an ALSA kernel driver for the DSP device.
>> >
>>
>> This is not a good approach of the general problem because a lot of
>> dsp has userspace
>> support and firmware and initialization is done by userspace library.
>> Can I change
>> the writei and readi of the dmix plugin and let use my dsp function?
>
> It's not trivial approach, either.  The key of dmix is the mmaped
> buffer shared among multiple clients, and the shared buffer may be
> altered at any time by any client.  This makes the post processing
> pretty difficult.
>
> If the post-processing can be applied in real time, one alternative
> approach would be to go through loopback device.  But it's also tricky
> and involves many round trips.
>

Yes I know about shared area mapping. Can you explain a bit more
the loopback device approch?

Michael

>
> Takashi