linux-kernel - Re: [alsa-devel] [PATCH 00/11] Add AXD Audio Processing IP driver

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Date:	Tue, 28 Oct 2014 17:04:07 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Qais Yousef <qais.yousef@...tec.com>,
	Clemens Ladisch <clemens@...isch.de>,
	linux-kernel@...r.kernel.org
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	alsa-devel@...a-project.org, Neil Jones <neil.jones@...tec.com>,
	Arnd Bergmann <arnd@...db.de>, devicetree@...r.kernel.org
Subject: Re: [alsa-devel] [PATCH 00/11] Add AXD Audio Processing IP driver

On 10/28/2014 04:33 PM, Qais Yousef wrote:
> On 10/28/2014 02:54 PM, Lars-Peter Clausen wrote:
>> On 10/28/2014 02:18 PM, Qais Yousef wrote:
>>> On 10/28/2014 11:55 AM, Clemens Ladisch wrote:
>>>> Qais Yousef wrote:
>>>>> AXD Audio Processing IP performs audio decoding, encoding, mixing,
>>>>> equalisation,
>>>>> synchronisation and playback.
>>>> What exactly do you mean with "synchronisation" and "playback"?
>>>
>>> Synchronisation refers to accurate audio playout relative to a master
>>> clock source including compensation of drift between the master clock
>>> source and the playout clock of the audio hardware. Hence allowing
>>> synchronised audio playout across multiple independent devices.
>>>
>>> Playback simple refers to the fact that AXD is capable of managing audio
>>> playout hardware like I2S and SPDIF interfaces.
>>>
>>>
>>>>> It doesn't fit in alsa subsystem but I Cced them to confirm.
>>>> ... because those two words sound like something that a sound card could
>>>> do.
>>>
>>> The problem mainly stems from the fact that we take a variety of
>>> compressed audio as input and we could perform audio encoding. The
>>> problem with the compressed audio is that the range of decoders and
>>> configuration supported in alsa is limited and there's no support for
>>> taking raw pcm and producing compressed output. I'm not an expert on
>>> alsa but when I looked it looked like there's more infra structure
>>> required.
>>>
>>> [...]
>>
>> This doesn't sound to different from any of the other supported audio
>> DSPs. ALSA seems to have 95% of what you need. And the missing 5% is
>> probably stuff that is not specific to your hardware but rather something
>> that other hardware will need as well. The framework is not set in stone
>> you can make modifications and add the features that are missing to make
>> your hardware work.
>>
>> E.g. look at sound/soc/intel/ for an example of a audio DSP.
>>
>> No integrating this into ALSA will quite likely result in a quite messy
>> situation for you on the long run.
>>
>> - Lars
>>
>>
>>
> OK thanks for the reference. To be honest it felt like 15-20% of the
> features are missing but I'll need to look at the specifics and
> judge/estimate better.

Even then you'd want ALSA integration. You (or your customers) will 
eventually want to hook some external devices up to the I2S or SPDIF ports. 
All the drivers for the external devices are implemented in ALSA/ASoC.

- Lars

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