On Mon, 28 Aug 2023 23:25:14 +0200, Wesley Cheng wrote: > > Hi Takashi, > > On 7/25/2023 2:27 AM, Takashi Iwai wrote: > > On Tue, 25 Jul 2023 11:15:11 +0200, > > 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@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(®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? > > > > That should be protected by the register_mutex. But there can be > > other races (see below :) > > > >> 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. > > > > Yes, we need to be more careful about binding. > > > > For example, in the current patch set, I see no way to prevent > > unloading snd-usb-audio-qmi module, and it allows user to cut off the > > stuff during operation, which may break things while the kernel is > > running the code of the unloaded module. You need to have a proper > > module refcount management for avoiding such a scenario. Most of > > drivers don't need it because ALSA core part already takes care of > > it. But in this case, it requires a manual adjustment. > > > > Sorry for the delayed response...this was routed to another folder, > and missed it. > > I see, that is a valid situation, so I think the best way to address > it is to do something like the following: > > static void __exit qc_usb_audio_offload_exit(void) > { > ... > snd_usb_unregister_platform_ops(); > for (idx = 0; idx < SNDRV_CARDS; idx++) > qc_usb_audio_offload_disconnect(uadev[idx].chip); > > We'll first unregister the platform ops, so that we get no further > connect/disconnect CBs, and then we'll forcefully ensure that all > devices are removed/cleaned. Part of the USB offload disconnect > sequence will also forcefully stop the offload path on the external > DSP by issuing a disconnect indication QMI message. > > Then we can safely clean up the rest of the resources. We do have > refcounting in place for several of the other structures, but I think > in the module exit case, we need to ensure the offload path is stopped > fully. Yes, the proper disconnection procedure is mandatory. In addition, you can have a module refcount increment upon the card binding, too. This may prevent the unexpected exit (although it's often still possible to unbind via sysfs). thanks, Takashi