Re: [PATCH v21 09/39] ASoC: qcom: qdsp6: Introduce USB AFE port to q6dsp

[Wesley Cheng]

Hi Pierre,

On 5/7/2024 1:37 PM, Pierre-Louis Bossart wrote:
> >   static struct snd_soc_dai_driver q6dsp_audio_fe_dais[] = {
> > +	{
> > +		.playback = {
> > +			.stream_name = "USB Playback",
> > +			.rates = SNDRV_PCM_RATE_8000 | SNDRV_PCM_RATE_11025 |
> > +					SNDRV_PCM_RATE_16000 | SNDRV_PCM_RATE_22050 |
> > +					SNDRV_PCM_RATE_32000 | SNDRV_PCM_RATE_44100 |
> > +					SNDRV_PCM_RATE_48000 | SNDRV_PCM_RATE_96000 |
> > +					SNDRV_PCM_RATE_192000,
> > +			.formats = SNDRV_PCM_FMTBIT_S16_LE | SNDRV_PCM_FMTBIT_S16_BE |
> > +					SNDRV_PCM_FMTBIT_U16_LE | SNDRV_PCM_FMTBIT_U16_BE |
> > +					SNDRV_PCM_FMTBIT_S24_LE | SNDRV_PCM_FMTBIT_S24_BE |
> > +					SNDRV_PCM_FMTBIT_U24_LE | SNDRV_PCM_FMTBIT_U24_BE,
> > +			.channels_min = 1,
> > +			.channels_max = 2,
> > +			.rate_min =	8000,
> > +			.rate_max = 192000,
> > +		},
> > +		.id = USB_RX,
> > +		.name = "USB_RX",
> > +	},
>
> Wait, is this saying you will have exactly one PCM device/FE DAI
> connected to the USB BE DAI exposed in patch 11?
>
> > +	SND_SOC_DAPM_MIXER("USB Mixer", SND_SOC_NOPM, 0, 0,
> > +			   usb_mixer_controls,
> > +			   ARRAY_SIZE(usb_mixer_controls)),
> > +
>
> And then what is the role of the USB mixer if you only have one input?
>
> I must be missing something.

Not sure if this is a QCOM specific implementation, but the way the DT 
is defined for the USB offload path is as follows:

	usb-dai-link {
		link-name = "USB Playback";

		cpu {
			sound-dai = <&q6afedai USB_RX>;
		};

		codec {
			sound-dai = <&usbdai USB_RX>;
		};

		platform {
			sound-dai = <&q6routing>;
		};
	};

Based on our DT parser helper API (qcom_snd_parse_of()) this isn't going 
to create a PCM device.  The PCM devices are created for nodes that 
don't have a codec and platform defined:

	mm1-dai-link {
		link-name = "MultiMedia1";
		cpu {
			sound-dai = <&q6asmdai  	MSM_FRONTEND_DAI_MULTIMEDIA1>;
		};
	};

The ASM path is the entity that defines the number of PCM devices that 
is created for the QC ASoC platform card, and is where the actual PCM 
data is sent over to the DSP.  So there could be several PCM devices 
that can use the USB BE DAI.

Thanks
Wesley Cheng