Re: [PATCH v13 50/53] ALSA: usb-audio: Allow for rediscovery of connected USB SND devices

["Amadeusz Sławiński" <amadeuszx.slawinski@linux.intel.com>]

On 2/3/2024 3:36 AM, Wesley Cheng wrote:
> In case of notifying SND platform drivers of connection events, some of
> these use cases, such as offloading, require an ASoC USB backend device to
> be initialized before the events can be handled.  If the USB backend device
> has not yet been probed, this leads to missing initial USB audio device
> connection events.
> 
> Expose an API that traverses the usb_chip array for connected devices, and
> to call the respective connection callback registered to the SND platform
> driver.
> 
> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
> ---
>   sound/usb/card.c                  | 19 +++++++++++++++++++
>   sound/usb/card.h                  |  2 ++
>   sound/usb/qcom/qc_audio_offload.c |  2 ++
>   3 files changed, 23 insertions(+)
> 
> diff --git a/sound/usb/card.c b/sound/usb/card.c
> index 11b827b7a2a5..995b2df676ab 100644
> --- a/sound/usb/card.c
> +++ b/sound/usb/card.c
> @@ -202,6 +202,25 @@ struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx,
>   }
>   EXPORT_SYMBOL_GPL(snd_usb_find_suppported_substream);
>   
> +/*
> + * in case the platform driver was not ready at the time of USB SND
> + * device connect, expose an API to discover all connected USB devices
> + * so it can populate any dependent resources/structures.
> + */
> +void snd_usb_rediscover_devices(void)
> +{
> +	int i;
> +
> +	mutex_lock(&register_mutex);
> +	for (i = 0; i < SNDRV_CARDS; i++) {
> +		if (usb_chip[i])
> +			if (platform_ops && platform_ops->connect_cb)
> +				platform_ops->connect_cb(usb_chip[i]);

if inside if, it can just be && or maybe move callback check before 
mutex lock and just return early if it is not present?

> +	}
> +	mutex_unlock(&register_mutex);
> +}
> +EXPORT_SYMBOL_GPL(snd_usb_rediscover_devices);
> +
>   /*
>    * disconnect streams
>    * called from usb_audio_disconnect()
> diff --git a/sound/usb/card.h b/sound/usb/card.h
> index 6d59995440c3..3a0d68f453a1 100644
> --- a/sound/usb/card.h
> +++ b/sound/usb/card.h
> @@ -222,11 +222,13 @@ int snd_usb_unregister_platform_ops(void);
>   #if IS_ENABLED(CONFIG_SND_USB_AUDIO)
>   struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx,
>   			struct snd_pcm_hw_params *params, int direction);
> +void snd_usb_rediscover_devices(void);
>   #else
>   static struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx,
>   			struct snd_pcm_hw_params *params, int direction)
>   {
>   	return NULL;
>   }
> +static void snd_usb_rediscover_devices(void) { }
>   #endif /* IS_ENABLED(CONFIG_SND_USB_AUDIO) */
>   #endif /* __USBAUDIO_CARD_H */
> diff --git a/sound/usb/qcom/qc_audio_offload.c b/sound/usb/qcom/qc_audio_offload.c
> index 08af82ec22ad..9b0f98600e58 100644
> --- a/sound/usb/qcom/qc_audio_offload.c
> +++ b/sound/usb/qcom/qc_audio_offload.c
> @@ -1867,6 +1867,8 @@ static int __init qc_usb_audio_offload_init(void)
>   	if (ret < 0)
>   		goto release_qmi;
>   
> +	snd_usb_rediscover_devices();
> +
>   	return 0;
>   
>   release_qmi:
>