Messages in this thread | | | Date | Tue, 15 Aug 2023 18:38:44 -0700 | Subject | Re: [PATCH v4 31/32] sound: usb: card: Allow for rediscovery of connected USB SND devices | From | Wesley Cheng <> |
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Hi Pierre,
On 7/25/2023 2:15 AM, Pierre-Louis Bossart wrote: > > > 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@quicinc.com> >> --- >> 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(®ister_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. >
The chip array entries are all populated and removed while under the register_mutex, so going over your race condition, we should see:
Thread#1: q6usb_component_probe() --> snd_soc_usb_add_port() --> snd_usb_rediscover_devices() --> mutex_lock(register_mutex)
Thread#2 --> usb_audio_disconnect() --> mutex_lock(register_mutex)
So either thread#1 or thread#2 will complete first. If
Thread#1 completes before thread#2: SOC USB will notify DPCM backend of the device connection. Shortly after, once thread#2 runs, we will get a disconnect event for the connected device.
Thread#2 completes before thread#1: Then during snd_usb_rediscover_devices() we won't notify of any connection for that particular chip index.
Thanks Wesley Cheng
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