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

[Pierre-Louis Bossart]

> > 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:
> >
> > /**
> > ÂÂ* struct snd_compr_caps - caps descriptor
> > ÂÂ* @codecs: pointer to array of codecs
> > ÂÂ* @direction: direction supported. Of type snd_compr_direction
> > ÂÂ* @min_fragment_size: minimum fragment supported by DSP
> > ÂÂ* @max_fragment_size: maximum fragment supported by DSP
> > ÂÂ* @min_fragments: min fragments supported by DSP
> > ÂÂ* @max_fragments: max fragments supported by DSP
> > ÂÂ* @num_codecs: number of codecs supported
> > ÂÂ* @reserved: reserved field
> > ÂÂ*/
> > struct snd_compr_caps {
> > ÂÂÂÂÂ__u32 num_codecs;
> > ÂÂÂÂÂ__u32 direction;
> > ÂÂÂÂÂ__u32 min_fragment_size;
> > ÂÂÂÂÂ__u32 max_fragment_size;
> > ÂÂÂÂÂ__u32 min_fragments;
> > ÂÂÂÂÂ__u32 max_fragments;
> > ÂÂÂÂÂ__u32 codecs[MAX_NUM_CODECS];
> > ÂÂÂÂÂ__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.