[Letux-kernel] Fixing Audio driver

H. Nikolaus Schaller hns at goldelico.com
Thu May 23 07:39:59 CEST 2019

> Am 23.05.2019 um 01:32 schrieb Michael Mrozek <EvilDragon at openpandora.org>:
> On Mi, 2019-05-22 at 19:53 +0200, H. Nikolaus Schaller wrote:
> Hi,
>>> ALSA reports 7 output devices. Accessing some of them leads to a
>>> kernel lock.
>> Some are additional sound cards for:
>> * bluetooth audio
>> * LTE modem voice channel
>> These do not work if the respective clock is missing. This is not yet
>> debugged and therefore never did work.
> I'm a bit puzzled here.
> On all my Linux systems, the bluetooth audio volume controls only pop
> up when I have a bluetooth audio device connected.
> Otherwise, they don't exist and are not listed in alsa or pulseaudio.

It may be possible to use the UART (HCI) to carry sound data. Our
module has a hardware PCM link which we map/present as a second sound card.

> The same should be the case for LTE unless the modem is initialized.
> Is the TWL even needed for Bluetooth Audio / LTE?

No. Every driver can register itself as a sound card. The twl6040 is one
with some (buggy) subchannels.

Then we add extra sound cards to access the hardware sound link for
Bluetooth and LTE.

A fourth sound card is the HDMI audio.

> I don't know how this works, but I always thought bluetooth audio is
> independent of the sound card in a system.

Here we have multiple sound cards.

> At least, from what I found on the internet, bluetooth audio is not
> affected by the sound card and it also works without any soundcard
> built-in.

This hardware based sound interface is intended to be used togehter
with AESS. Then, you can configure AESS to route LTE voice channel
to Bluetooth and idle the main processor. This goes far beyond the
simple case of a Bluetooth headset on a PC.

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