linux-kernel - Re: [PATCH v28 30/32] ALSA: usb-audio: Add USB offload route kcontrol

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Message-ID: <8bb65adc-e995-443e-80c9-36e9b5d8eee3@linux.intel.com>
Date: Wed, 25 Sep 2024 16:54:28 +0200
From: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To: Wesley Cheng <quic_wcheng@...cinc.com>, 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, tiwai@...e.com, krzk+dt@...nel.org,
 Thinh.Nguyen@...opsys.com, bgoswami@...cinc.com, robh@...nel.org,
 gregkh@...uxfoundation.org
Cc: 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, alsa-devel@...a-project.org
Subject: Re: [PATCH v28 30/32] ALSA: usb-audio: Add USB offload route kcontrol




> +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;
> +	}

well this invalidates again what I understood from the last patch and
goes back to what I understood initially: the error code is never
returned to higher levels - when offload is not supported the kcontrol
values are encoded to the -1 magic value.

> +	return 0;

and this begs the question if this helper should return a void value.

> +}
> +
> +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];

that's quite a magic value.

> +	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;
> +}