Re: [PATCH v13 48/53] ALSA: usb-audio: mixer: Add USB offloading mixer control

Takashi Iwai
2024-02-06T12:57:49Z

On Sat, 03 Feb 2024 03:36:40 +0100,
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
> index is associated with the ASoC platform card supporting offloading. In
> the USB audio offloading framework, the ASoC BE DAI link is the entity
> responsible for registering to the SOC USB layer. SOC USB will expose more
> details about the current offloading status, which includes the USB sound
> card and USB PCM device indexes currently being used.
>
> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>

The concept is understandable, but the control element name ("SNDUSB
OFFLD playback available") looks non-intrusive and non-conformant.
Use a bit more understandable name instead.

This provides a card number where the offload driver is bound, and the
name should indicate something about that.

Also, about the implementation:

> +static int
> +snd_usb_offload_create_mixer(struct usb_mixer_interface *mixer,
> + const struct snd_kcontrol_new *new_kctl)
> +{
> + struct snd_kcontrol *kctl;
> + struct usb_mixer_elem_info *elem;
> +
> + elem = kzalloc(sizeof(struct usb_mixer_elem_info), GFP_KERNEL);
> + if (!elem)
> + return -ENOMEM;
> +
> + elem->head.mixer = mixer;
> + elem->val_type = USB_MIXER_S32;
> + elem->control = 0;
> + elem->head.id = 0;
> + elem->channels = 1;
> +
> + kctl = snd_ctl_new1(new_kctl, elem);
> + if (!kctl) {
> + kfree(elem);
> + return -ENOMEM;
> + }
> + kctl->private_free = snd_usb_mixer_elem_free;
> +
> + return snd_usb_mixer_add_control(&elem->head, kctl);

This control has almost little to do with the standard USB interface,
and it'll be much simpler if you create a raw control element.
Pass the bus or the sysdev to private_data, and that's all you need in
the get callback.

Also, don't forget to set the proper access bits (it's read-only).

thanks,

Takashi