linux-kernel - Re: [PATCH v36 30/31] ALSA: usb-audio: qcom: Add USB offload route kcontrol

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Message-ID: <Z-KU_o_LE3PO6J2y@linaro.org>
Date: Tue, 25 Mar 2025 12:35:26 +0100
From: Stephan Gerhold <stephan.gerhold@...aro.org>
To: Wesley Cheng <quic_wcheng@...cinc.com>
Cc: srinivas.kandagatla@...aro.org, mathias.nyman@...el.com, perex@...ex.cz,
	conor+dt@...nel.org, dmitry.torokhov@...il.com, corbet@....net,
	broonie@...nel.org, lgirdwood@...il.com, krzk+dt@...nel.org,
	pierre-louis.bossart@...ux.intel.com, Thinh.Nguyen@...opsys.com,
	tiwai@...e.com, robh@...nel.org, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	linux-sound@...r.kernel.org, linux-input@...r.kernel.org,
	linux-usb@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH v36 30/31] ALSA: usb-audio: qcom: Add USB offload route
 kcontrol

On Tue, Mar 18, 2025 at 05:51:40PM -0700, Wesley Cheng wrote:
> In order to allow userspace/applications know about USB offloading status,
> expose a sound kcontrol that fetches information about which sound card
> and PCM index the USB device is mapped to for supporting offloading.  In
> the USB audio offloading framework, the ASoC BE DAI link is the entity
> responsible for registering to the SOC USB layer.
> 
> It is expected for the USB SND offloading driver to add the kcontrol to the
> sound card associated with the USB audio device.  An example output would
> look like:
> 
> tinymix -D 1 get 'USB Offload Playback Route PCM#0'
> -1, -1 (range -1->255)
> 
> This example signifies that there is no mapped ASoC path available for the
> USB SND device.
> 
> tinymix -D 1 get 'USB Offload Playback Route PCM#0'
> 0, 0 (range -1->255)
> 
> This example signifies that the offload path is available over ASoC sound
> card index#0 and PCM device#0.
> 
> The USB offload kcontrol will be added in addition to the existing
> kcontrols identified by the USB SND mixer.  The kcontrols used to modify
> the USB audio device specific parameters are still valid and expected to be
> used.  These parameters are not mirrored to the ASoC subsystem.
> 
> Signed-off-by: Wesley Cheng <quic_wcheng@...cinc.com>
> ---
>  sound/usb/Kconfig                  |  10 ++
>  sound/usb/qcom/Makefile            |   4 +
>  sound/usb/qcom/mixer_usb_offload.c | 158 +++++++++++++++++++++++++++++
>  sound/usb/qcom/mixer_usb_offload.h |  17 ++++
>  sound/usb/qcom/qc_audio_offload.c  |   2 +
>  5 files changed, 191 insertions(+)
>  create mode 100644 sound/usb/qcom/mixer_usb_offload.c
>  create mode 100644 sound/usb/qcom/mixer_usb_offload.h
> 
> diff --git a/sound/usb/Kconfig b/sound/usb/Kconfig
> index 6daa551738da..7d8833945711 100644
> --- a/sound/usb/Kconfig
> +++ b/sound/usb/Kconfig
> @@ -176,9 +176,19 @@ config SND_BCD2000
>  	  To compile this driver as a module, choose M here: the module
>  	  will be called snd-bcd2000.
>  
> +config SND_USB_QC_OFFLOAD_MIXER
> +	tristate "Qualcomm USB Audio Offload mixer control"

This looks like a "bool" and not a "tristate", since the ifneq in the
Makefile below ignores whether this is a "y" or "m".

> +	help
> +	 Say Y to enable the Qualcomm USB audio offloading mixer controls.
> +	 This exposes an USB offload capable kcontrol to signal to
> +	 applications about which platform sound card can support USB
> +	 audio offload.  The returning values specify the mapped ASoC card
> +	 and PCM device the USB audio device is associated to.
> +
>  config SND_USB_AUDIO_QMI
>  	tristate "Qualcomm Audio Offload driver"
>  	depends on QCOM_QMI_HELPERS && SND_USB_AUDIO && USB_XHCI_SIDEBAND && SND_SOC_USB
> +	select SND_USB_OFFLOAD_MIXER

And I think "SND_USB_OFFLOAD_MIXER" (without _QC suffix) doesn't exist
anymore after v30? I see there was some discussion around that there.
Is this supposed to be "select SND_USB_QC_OFFLOAD_MIXER"?

If yes, isn't this option always selected if SND_USB_AUDIO_QMI is
enabled? In that case you could just drop the config option...

>  	help
>  	  Say Y here to enable the Qualcomm USB audio offloading feature.
>  
> diff --git a/sound/usb/qcom/Makefile b/sound/usb/qcom/Makefile
> index 1eb51160e2e5..1efe1b90be7a 100644
> --- a/sound/usb/qcom/Makefile
> +++ b/sound/usb/qcom/Makefile
> @@ -1,2 +1,6 @@
>  snd-usb-audio-qmi-y := usb_audio_qmi_v01.o qc_audio_offload.o
>  obj-$(CONFIG_SND_USB_AUDIO_QMI) += snd-usb-audio-qmi.o
> +
> +ifneq ($(CONFIG_SND_USB_QC_OFFLOAD_MIXER),)
> +snd-usb-audio-qmi-y += mixer_usb_offload.o
> +endif

Thanks,
Stephan