linux-kernel - Re: [PATCH 10/11] ASoC: qdsp6-dai: add gapless support

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Date:   Wed, 8 Jul 2020 11:58:21 -0500
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        broonie@...nel.org
Cc:     alsa-devel@...a-project.org, ckeepax@...nsource.cirrus.com,
        lgirdwood@...il.com, linux-kernel@...r.kernel.org, tiwai@...e.com,
        vkoul@...nel.org
Subject: Re: [PATCH 10/11] ASoC: qdsp6-dai: add gapless support


>> right, but the key point is 'switch between streams'. That means a 
>> more complex/capable implementation that should be advertised as such 
>> to applications. This is not the default behavior assumed initially: 
>> to allow for minimal implementations in memory-constrained devices, we 
>> assumed gapless was supported with a single decoder.
>>
>> Maybe the right way to do this is extend the snd_compr_caps structure:
>>
>> /**
>>   * struct snd_compr_caps - caps descriptor
>>   * @codecs: pointer to array of codecs
>>   * @direction: direction supported. Of type snd_compr_direction
>>   * @min_fragment_size: minimum fragment supported by DSP
>>   * @max_fragment_size: maximum fragment supported by DSP
>>   * @min_fragments: min fragments supported by DSP
>>   * @max_fragments: max fragments supported by DSP
>>   * @num_codecs: number of codecs supported
>>   * @reserved: reserved field
>>   */
>> struct snd_compr_caps {
>>      __u32 num_codecs;
>>      __u32 direction;
>>      __u32 min_fragment_size;
>>      __u32 max_fragment_size;
>>      __u32 min_fragments;
>>      __u32 max_fragments;
>>      __u32 codecs[MAX_NUM_CODECS];
>>      __u32 reserved[11];
>> } __attribute__((packed, aligned(4)));
>>
>>
>> and use a reserved field to provide info on capabilities, and filter 
>> the set_codec_params() addition based this capability - i.e. return 
>> -ENOTSUP in 'traditional' implementations based on a single 
>> 'stream'/decoder instance.

I think this is also what Mark was referring to earlier.

> Sounds good!
> I will give it a go and see how it ends up!

Glad to see this discussion progressing.

We may also want to document the 3 possible ways of supporting gapless 
playback while we are at it:
a) with the existing single decoder assumption
b) with your suggested solution with a switch at the DSP level
c) with 2 streams at the userspace level and a switch/x-fade at the DSP 
level - which may simplify userspace quite a bit and was the initial 
design in a non-Linux OS.