Re: [PATCH v26 31/33] ALSA: usb-audio: Add USB offload route kcontrol

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On 8/29/24 21:41, 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:

this is very odd, the offloading driver adds a control to another card?

That seems like a rather important layering violation.

I thought it was done the other way, the USB audio card created a
control that would point to the other card/device.


> 
> 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.

but that control would not even exist if the ASoC-based driver isn't probed.

It's become really hard to follow, I've been on all this for 1.5hrs and
losing track of the design.

> 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.
> 
> Signed-off-by: Wesley Cheng <quic_wcheng@xxxxxxxxxxx>
> ---
>  sound/usb/Kconfig                 |  10 +++
>  sound/usb/Makefile                |   2 +
>  sound/usb/mixer_usb_offload.c     | 102 ++++++++++++++++++++++++++++++
>  sound/usb/mixer_usb_offload.h     |  17 +++++
>  sound/usb/qcom/Makefile           |   2 +-
>  sound/usb/qcom/qc_audio_offload.c |   2 +
>  6 files changed, 134 insertions(+), 1 deletion(-)
>  create mode 100644 sound/usb/mixer_usb_offload.c
>  create mode 100644 sound/usb/mixer_usb_offload.h
> 
> diff --git a/sound/usb/Kconfig b/sound/usb/Kconfig
> index 5cc3eaf53e6b..e3fbf9541d0b 100644
> --- a/sound/usb/Kconfig
> +++ b/sound/usb/Kconfig
> @@ -176,10 +176,20 @@ config SND_BCD2000
>  	  To compile this driver as a module, choose M here: the module
>  	  will be called snd-bcd2000.
>  
> +config SND_USB_OFFLOAD_MIXER
> +	tristate "USB Audio Offload mixer control"
> +	help
> +	 Say Y to enable the 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_PCM
> +	select SND_USB_OFFLOAD_MIXER
>  	help
>  	  Say Y here to enable the Qualcomm USB audio offloading feature.
>  
> diff --git a/sound/usb/Makefile b/sound/usb/Makefile
> index 54a06a2f73ca..2f19f854944c 100644
> --- a/sound/usb/Makefile
> +++ b/sound/usb/Makefile
> @@ -36,3 +36,5 @@ obj-$(CONFIG_SND_USB_US122L) += snd-usbmidi-lib.o
>  
>  obj-$(CONFIG_SND) += misc/ usx2y/ caiaq/ 6fire/ hiface/ bcd2000/ qcom/
>  obj-$(CONFIG_SND_USB_LINE6)	+= line6/
> +
> +obj-$(CONFIG_SND_USB_OFFLOAD_MIXER) += mixer_usb_offload.o
> diff --git a/sound/usb/mixer_usb_offload.c b/sound/usb/mixer_usb_offload.c
> new file mode 100644
> index 000000000000..16e2fd634684
> --- /dev/null
> +++ b/sound/usb/mixer_usb_offload.c
> @@ -0,0 +1,102 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (c) 2022-2024 Qualcomm Innovation Center, Inc. All rights reserved.
> + */
> +
> +#include <linux/usb.h>
> +
> +#include <sound/core.h>
> +#include <sound/control.h>
> +#include <sound/soc-usb.h>
> +
> +#include "usbaudio.h"
> +#include "card.h"
> +#include "helper.h"
> +#include "mixer.h"
> +
> +#include "mixer_usb_offload.h"
> +
> +#define PCM_IDX(n)  ((n) & 0xffff)
> +#define CARD_IDX(n) ((n) >> 16)
> +
> +static int
> +snd_usb_offload_route_get(struct snd_kcontrol *kcontrol,
> +			  struct snd_ctl_elem_value *ucontrol)
> +{
> +	struct device *sysdev = snd_kcontrol_chip(kcontrol);
> +	int ret;
> +
> +	ret = snd_soc_usb_update_offload_route(sysdev,
> +					       CARD_IDX(kcontrol->private_value),
> +					       PCM_IDX(kcontrol->private_value),
> +					       SNDRV_PCM_STREAM_PLAYBACK,
> +					       ucontrol->value.integer.value);
> +	if (ret < 0) {
> +		ucontrol->value.integer.value[0] = -1;
> +		ucontrol->value.integer.value[1] = -1;
> +	}
> +
> +	return 0;
> +}
> +
> +static int snd_usb_offload_route_info(struct snd_kcontrol *kcontrol,
> +				      struct snd_ctl_elem_info *uinfo)
> +{
> +	uinfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
> +	uinfo->count = 2;
> +	uinfo->value.integer.min = -1;
> +	/* Arbitrary max value, as there is no 'limit' on number of PCM devices */
> +	uinfo->value.integer.max = 0xff;
> +
> +	return 0;
> +}
> +
> +static struct snd_kcontrol_new snd_usb_offload_mapped_ctl = {
> +	.iface = SNDRV_CTL_ELEM_IFACE_CARD,
> +	.access = SNDRV_CTL_ELEM_ACCESS_READ,
> +	.info = snd_usb_offload_route_info,
> +	.get = snd_usb_offload_route_get,
> +};
> +
> +/**
> + * snd_usb_offload_create_ctl() - Add USB offload bounded mixer
> + * @chip - USB SND chip device
> + *
> + * Creates a sound control for a USB audio device, so that applications can
> + * query for if there is an available USB audio offload path, and which
> + * card is managing it.
> + */
> +int snd_usb_offload_create_ctl(struct snd_usb_audio *chip)
> +{
> +	struct usb_device *udev = chip->dev;
> +	struct snd_kcontrol_new *chip_kctl;
> +	struct snd_usb_substream *subs;
> +	struct snd_usb_stream *as;
> +	char ctl_name[37];
> +	int ret;
> +
> +	list_for_each_entry(as, &chip->pcm_list, list) {
> +		subs = &as->substream[SNDRV_PCM_STREAM_PLAYBACK];
> +		if (!subs->ep_num)
> +			continue;
> +
> +		chip_kctl = &snd_usb_offload_mapped_ctl;
> +		chip_kctl->count = 1;
> +		/*
> +		 * Store the associated USB SND card number and PCM index for
> +		 * the kctl.
> +		 */
> +		chip_kctl->private_value = as->pcm_index |
> +					  chip->card->number << 16;
> +		sprintf(ctl_name, "USB Offload Playback Route PCM#%d",
> +			as->pcm_index);
> +		chip_kctl->name = ctl_name;
> +		ret = snd_ctl_add(chip->card, snd_ctl_new1(chip_kctl,
> +				  udev->bus->sysdev));
> +		if (ret < 0)
> +			break;
> +	}
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(snd_usb_offload_create_ctl);
> diff --git a/sound/usb/mixer_usb_offload.h b/sound/usb/mixer_usb_offload.h
> new file mode 100644
> index 000000000000..3f6e2a8b19c8
> --- /dev/null
> +++ b/sound/usb/mixer_usb_offload.h
> @@ -0,0 +1,17 @@
> +/* SPDX-License-Identifier: GPL-2.0
> + *
> + * Copyright (c) 2022-2024 Qualcomm Innovation Center, Inc. All rights reserved.
> + */
> +
> +#ifndef __USB_OFFLOAD_MIXER_H
> +#define __USB_OFFLOAD_MIXER_H
> +
> +#if IS_ENABLED(CONFIG_SND_USB_OFFLOAD_MIXER)
> +int snd_usb_offload_create_ctl(struct snd_usb_audio *chip);
> +#else
> +static inline int snd_usb_offload_create_ctl(struct snd_usb_audio *chip)
> +{
> +	return 0;
> +}
> +#endif
> +#endif /* __USB_OFFLOAD_MIXER_H */
> diff --git a/sound/usb/qcom/Makefile b/sound/usb/qcom/Makefile
> index a81c9b28d484..4005e8391ab9 100644
> --- a/sound/usb/qcom/Makefile
> +++ b/sound/usb/qcom/Makefile
> @@ -1,2 +1,2 @@
>  snd-usb-audio-qmi-objs := usb_audio_qmi_v01.o qc_audio_offload.o
> -obj-$(CONFIG_SND_USB_AUDIO_QMI) += snd-usb-audio-qmi.o
> \ No newline at end of file
> +obj-$(CONFIG_SND_USB_AUDIO_QMI) += snd-usb-audio-qmi.o
> diff --git a/sound/usb/qcom/qc_audio_offload.c b/sound/usb/qcom/qc_audio_offload.c
> index a7ad15404fd1..5b9262a116be 100644
> --- a/sound/usb/qcom/qc_audio_offload.c
> +++ b/sound/usb/qcom/qc_audio_offload.c
> @@ -36,6 +36,7 @@
>  #include "../helper.h"
>  #include "../pcm.h"
>  #include "../power.h"
> +#include "../mixer_usb_offload.h"
>  
>  #include "usb_audio_qmi_v01.h"
>  
> @@ -1703,6 +1704,7 @@ static void qc_usb_audio_offload_probe(struct snd_usb_audio *chip)
>  		sdev->card_idx = chip->card->number;
>  		sdev->chip_idx = chip->index;
>  
> +		snd_usb_offload_create_ctl(chip);
>  		snd_soc_usb_connect(usb_get_usb_backend(udev), sdev);
>  	}
>  




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