Re: [PATCH v9 33/34] ALSA: usb-audio: Allow for rediscovery of connected USB SND devices

[Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>]

On 10/17/23 15:01, 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@xxxxxxxxxxx>
> ---
>  sound/usb/card.c | 19 +++++++++++++++++++
>  sound/usb/card.h |  2 ++
>  2 files changed, 21 insertions(+)
> 
> diff --git a/sound/usb/card.c b/sound/usb/card.c
> index 88f431917c15..7d3f470754ca 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.

it's really unclear how the "platform driver was not ready at the time
of USB SND device connect" condition can be detected. It's my
understanding that we should never assume any specific order or timing
on platform driver probes. And things should also work if a driver is
unbound.

> + */
> +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]);
> +	}
> +	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 01f7e10f30f4..c0aeda17ed69 100644
> --- a/sound/usb/card.h
> +++ b/sound/usb/card.h
> @@ -221,11 +221,13 @@ 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 */