Re: [PATCH v4 31/32] sound: usb: card: Allow for rediscovery of connected USB SND devices

[Pierre-Louis Bossart]

On 7/25/23 04:34, 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 365f6d978608..27a89aaa0bf3 100644
> --- a/sound/usb/card.c
> +++ b/sound/usb/card.c
> @@ -170,6 +170,25 @@ 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]);

what happens if the USB device is removed while the platform device adds
a port?

This sounds super-racy to me. It's the same set of problems we're having
between audio and display/DRM, I would be surprised if this function
dealt with all corner cases of insertion/removal, bind/unbind.


> +	}
> +	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 db735680d155..dfd241334e3d 100644
> --- a/sound/usb/card.h
> +++ b/sound/usb/card.h
> @@ -219,6 +219,7 @@ int snd_usb_register_platform_ops(struct snd_usb_platform_ops *ops);
>  int snd_usb_unregister_platform_ops(void);
>  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 int snd_usb_register_platform_ops(struct snd_usb_platform_ops *ops)
>  {
> @@ -235,5 +236,6 @@ static struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx,
>  {
>  	return NULL;
>  }
> +void snd_usb_rediscover_devices(void) { }
>  #endif /* IS_ENABLED(CONFIG_SND_USB_AUDIO) */
>  #endif /* __USBAUDIO_CARD_H */