Dne 23. 12. 21 v 9:18 Pavel Hofman napsal(a):

Hi Takashi,

I am working on stopping alsa streams of audio USB gadget when USB host stops capture/playback/USB cable unplugged.

For capture I used code from AK4114 SPDIF receiver https://elixir.bootlin.com/linux/latest/source/sound/i2c/other/ak4114.c#L590:

static void stop_substream(struct uac_rtd_params *prm) {     unsigned long _flags;     struct snd_pcm_substream *substream;

substream = prm->ss;     if (substream) {         snd_pcm_stream_lock_irqsave(substream, _flags);         if (snd_pcm_running(substream))             // TODO - correct handling for playback substream?             snd_pcm_stop(substream, SNDRV_PCM_STATE_DRAINING);         snd_pcm_stream_unlock_irqrestore(substream, _flags);     } }

For setup I found calling snd_pcm_stop(substream, SNDRV_PCM_STATE_SETUP) (https://elixir.bootlin.com/linux/latest/source/drivers/staging/vc04_services...)  Or for both capture and playback using SNDRV_PCM_STATE_DISCONNECTED (https://elixir.bootlin.com/linux/latest/source/sound/core/pcm.c#L1103).

Or perhaps using snd_pcm_dev_disconnect(dev) or snd_pcm_drop(substream)?

Please what is the recommended way?

Please can I ask for expert view on this issue? E.g. in SoX stopping the stream with SNDRV_PCM_STATE_SETUP/SNDRV_PCM_STATE_DRAINING does not stop the application, while with SNDRV_PCM_STATE_DISCONNECTED SoX exits with non-recoverable status. I am considering implementing both methods and letting users choose their suitable snd_pcm_stop operation (none (default)/SETUP-DRAINING/DISCONNECTED) for the two events (host playback/capture stop, cable disconnection) with a configfs param. Would this make sense?

Thanks a lot in advance,

Pavel.

Dne 03. 01. 22 v 10:10 Jaroslav Kysela napsal(a):

On 03. 01. 22 9:22, Pavel Hofman wrote:

...

Dne 23. 12. 21 v 9:18 Pavel Hofman napsal(a):

...

Hi Takashi,

I am working on stopping alsa streams of audio USB gadget when USB host stops capture/playback/USB cable unplugged.

For capture I used code from AK4114 SPDIF receiver https://elixir.bootlin.com/linux/latest/source/sound/i2c/other/ak4114.c#L590:

static void stop_substream(struct uac_rtd_params *prm) {       unsigned long _flags;       struct snd_pcm_substream *substream;

substream = prm->ss;       if (substream) {           snd_pcm_stream_lock_irqsave(substream, _flags);           if (snd_pcm_running(substream))               // TODO - correct handling for playback substream?               snd_pcm_stop(substream, SNDRV_PCM_STATE_DRAINING);           snd_pcm_stream_unlock_irqrestore(substream, _flags);       } }

For setup I found calling snd_pcm_stop(substream, SNDRV_PCM_STATE_SETUP) (https://elixir.bootlin.com/linux/latest/source/drivers/staging/vc04_services...)

Or for both capture and playback using SNDRV_PCM_STATE_DISCONNECTED (https://elixir.bootlin.com/linux/latest/source/sound/core/pcm.c#L1103).

Or perhaps using snd_pcm_dev_disconnect(dev) or snd_pcm_drop(substream)?

Please what is the recommended way?

Please can I ask for expert view on this issue? E.g. in SoX stopping the stream with SNDRV_PCM_STATE_SETUP/SNDRV_PCM_STATE_DRAINING does not stop the application, while with SNDRV_PCM_STATE_DISCONNECTED SoX exits with non-recoverable status. I am considering implementing both methods and letting users choose their suitable snd_pcm_stop operation (none (default)/SETUP-DRAINING/DISCONNECTED) for the two events (host playback/capture stop, cable disconnection) with a configfs param. Would this make sense?

The disconnection state is unrecoverable. It's expected that the device will be freed and cannot be reused.

If you expect to keep the PCM device, we should probably introduce a new function which puts the device to the SNDRV_PCM_STATE_OPEN state. In this state, all I/O routines will return -EBADFD for the applications, so they should close or re-initialize the PCM device completely.

https://elixir.bootlin.com/linux/latest/source/sound/core/pcm_native.c#L794

The fact is that after closing the USB host can re-open the device with different samplerate (and perhaps later on with different channels count/sample size). That would hint at the need to re-initialize the gadget side before opening anyway.

As of keeping the device - it's likely some use cases would prefer keeping the device, to minimize the operations needed to react to the host-side playback/capture start.

A function you describe would make sense for this. IMO from the gadget POW there is no difference between the host stopping playback/capture and cable disconnection, in both cases the data stream is stopped and next stream can have entirely different parameters. Maybe the gadget configfs parameter could only toggle between no action (i.e. current situation) and the new alsa function stopping the stream.

Jaroslav, please can you draft such a function? Perhaps both changes could make it to 5.17.

Thanks a lot,

Pavel.

On 03. 01. 22 13:15, Takashi Iwai wrote:

On Mon, 03 Jan 2022 12:32:53 +0100, Pavel Hofman wrote:

...

Dne 03. 01. 22 v 10:10 Jaroslav Kysela napsal(a):

...

On 03. 01. 22 9:22, Pavel Hofman wrote:

...

Dne 23. 12. 21 v 9:18 Pavel Hofman napsal(a):

...

Hi Takashi,

I am working on stopping alsa streams of audio USB gadget when USB host stops capture/playback/USB cable unplugged.

For capture I used code from AK4114 SPDIF receiver https://elixir.bootlin.com/linux/latest/source/sound/i2c/other/ak4114.c#L590:

static void stop_substream(struct uac_rtd_params *prm) {       unsigned long _flags;       struct snd_pcm_substream *substream;

substream = prm->ss;       if (substream) {           snd_pcm_stream_lock_irqsave(substream, _flags);           if (snd_pcm_running(substream))               // TODO - correct handling for playback substream?               snd_pcm_stop(substream, SNDRV_PCM_STATE_DRAINING);           snd_pcm_stream_unlock_irqrestore(substream, _flags);       } }

For setup I found calling snd_pcm_stop(substream, SNDRV_PCM_STATE_SETUP) (https://elixir.bootlin.com/linux/latest/source/drivers/staging/vc04_services...)

Or for both capture and playback using SNDRV_PCM_STATE_DISCONNECTED (https://elixir.bootlin.com/linux/latest/source/sound/core/pcm.c#L1103).

Or perhaps using snd_pcm_dev_disconnect(dev) or snd_pcm_drop(substream)?

Please what is the recommended way?

Please can I ask for expert view on this issue? E.g. in SoX stopping the stream with SNDRV_PCM_STATE_SETUP/SNDRV_PCM_STATE_DRAINING does not stop the application, while with SNDRV_PCM_STATE_DISCONNECTED SoX exits with non-recoverable status. I am considering implementing both methods and letting users choose their suitable snd_pcm_stop operation (none (default)/SETUP-DRAINING/DISCONNECTED) for the two events (host playback/capture stop, cable disconnection) with a configfs param. Would this make sense?

The disconnection state is unrecoverable. It's expected that the device will be freed and cannot be reused.

If you expect to keep the PCM device, we should probably introduce a new function which puts the device to the SNDRV_PCM_STATE_OPEN state. In this state, all I/O routines will return -EBADFD for the applications, so they should close or re-initialize the PCM device completely.

https://elixir.bootlin.com/linux/latest/source/sound/core/pcm_native.c#L794

The fact is that after closing the USB host can re-open the device with different samplerate (and perhaps later on with different channels count/sample size). That would hint at the need to re-initialize the gadget side before opening anyway.

As of keeping the device - it's likely some use cases would prefer keeping the device, to minimize the operations needed to react to the host-side playback/capture start.

A function you describe would make sense for this. IMO from the gadget POW there is no difference between the host stopping playback/capture and cable disconnection, in both cases the data stream is stopped and next stream can have entirely different parameters. Maybe the gadget configfs parameter could only toggle between no action (i.e. current situation) and the new alsa function stopping the stream.

Jaroslav, please can you draft such a function? Perhaps both changes could make it to 5.17.

(Sorry for the delayed response, as I've been on vacation and now catching up the huge pile of backlogs...)

About the change to keep PCM OPEN state: I'm afraid that the disconnection in the host side may happen at any time, and keeping the state OPEN would confuse the things if the host is indeed unrecoverable.

I don't think so. The SNDRV_PCM_IOCTL_HW_PARAMS must be issued by the application (in the PCM_OPEN state) and if the USB bus connection is no longer active, it may fail. We can distinguish between host -> device disconnection and device -> host one. It is not really a similar thing.

I think that the idea was to avoid to re-build the whole card / device structure for the fixed device allocation.

Pavel, if the USB host is not connected to the gadget, where the playback PCM device fails now ? Is the PCM device created or not ?

Jaroslav

Dne 03. 01. 22 v 13:15 Takashi Iwai napsal(a):

On Mon, 03 Jan 2022 12:32:53 +0100, Pavel Hofman wrote:

...

Dne 03. 01. 22 v 10:10 Jaroslav Kysela napsal(a):

...

On 03. 01. 22 9:22, Pavel Hofman wrote:

...

Dne 23. 12. 21 v 9:18 Pavel Hofman napsal(a):

...

Hi Takashi,

I am working on stopping alsa streams of audio USB gadget when USB host stops capture/playback/USB cable unplugged.

For capture I used code from AK4114 SPDIF receiver https://elixir.bootlin.com/linux/latest/source/sound/i2c/other/ak4114.c#L590:

static void stop_substream(struct uac_rtd_params *prm) {       unsigned long _flags;       struct snd_pcm_substream *substream;

substream = prm->ss;       if (substream) {           snd_pcm_stream_lock_irqsave(substream, _flags);           if (snd_pcm_running(substream))               // TODO - correct handling for playback substream?               snd_pcm_stop(substream, SNDRV_PCM_STATE_DRAINING);           snd_pcm_stream_unlock_irqrestore(substream, _flags);       } }

For setup I found calling snd_pcm_stop(substream, SNDRV_PCM_STATE_SETUP) (https://elixir.bootlin.com/linux/latest/source/drivers/staging/vc04_services...)

Or for both capture and playback using SNDRV_PCM_STATE_DISCONNECTED (https://elixir.bootlin.com/linux/latest/source/sound/core/pcm.c#L1103).

Or perhaps using snd_pcm_dev_disconnect(dev) or snd_pcm_drop(substream)?

Please what is the recommended way?

Please can I ask for expert view on this issue? E.g. in SoX stopping the stream with SNDRV_PCM_STATE_SETUP/SNDRV_PCM_STATE_DRAINING does not stop the application, while with SNDRV_PCM_STATE_DISCONNECTED SoX exits with non-recoverable status. I am considering implementing both methods and letting users choose their suitable snd_pcm_stop operation (none (default)/SETUP-DRAINING/DISCONNECTED) for the two events (host playback/capture stop, cable disconnection) with a configfs param. Would this make sense?

The disconnection state is unrecoverable. It's expected that the device will be freed and cannot be reused.

If you expect to keep the PCM device, we should probably introduce a new function which puts the device to the SNDRV_PCM_STATE_OPEN state. In this state, all I/O routines will return -EBADFD for the applications, so they should close or re-initialize the PCM device completely.

https://elixir.bootlin.com/linux/latest/source/sound/core/pcm_native.c#L794

The fact is that after closing the USB host can re-open the device with different samplerate (and perhaps later on with different channels count/sample size). That would hint at the need to re-initialize the gadget side before opening anyway.

As of keeping the device - it's likely some use cases would prefer keeping the device, to minimize the operations needed to react to the host-side playback/capture start.

A function you describe would make sense for this. IMO from the gadget POW there is no difference between the host stopping playback/capture and cable disconnection, in both cases the data stream is stopped and next stream can have entirely different parameters. Maybe the gadget configfs parameter could only toggle between no action (i.e. current situation) and the new alsa function stopping the stream.

Jaroslav, please can you draft such a function? Perhaps both changes could make it to 5.17.

(Sorry for the delayed response, as I've been on vacation and now catching up the huge pile of backlogs...)

About the change to keep PCM OPEN state: I'm afraid that the disconnection in the host side may happen at any time, and keeping the state OPEN would confuse the things if the host is indeed unrecoverable. That said, from the safety POV, the complete card-level disconnection would make sense, which has, of course, a clear downside for the smooth transition in the application as you described above. But most applications should handle such a disconnection in anyway for the normal USB-audio devices that face more or less the same problem.

Thanks for your valuable insight. IMO a major difference between host and gadget side is that when the USB cable gets disconnected, the host alsa device disappears, while the (existing) gadget alsa device just stops delivering/consuming samples (with the blocking read/write timing out, eventually).

I sent a series of RFC patches to linux-usb which handle signalling that the host started or stopped (+ cable disconnection) to the gadget side via alsa ctl + snd_ctl_notify, as sort of a side channel. A user-space example implementation of handling the ctl events on the gadget side is https://github.com/pavhofman/gaudio_ctl . The feature of stopping the alsa substreams is a mechanism e.g. for setups where the application keeps polling the alsa device whether it can be opened. Also sending a signal (term, hup, usr1, ...) to an app which is blocked on alsa read/write is more difficult without interrupting the blocking wait with snd_pcm_close by the gadget device.

I am not sure the stop should "destroy" the whole device, it seems a bit of overkill to me. Really just closing the device cleanly so that it must be again initialized with new hw_params (which can change between starts) would do, perhaps.

Thanks a lot,

Pavel.

On Mon, Jan 03, 2022 at 01:54:13PM +0100, Pavel Hofman wrote:

Dne 03. 01. 22 v 13:28 Jaroslav Kysela napsal(a):

...

On 03. 01. 22 13:15, Takashi Iwai wrote:

...

On Mon, 03 Jan 2022 12:32:53 +0100, Pavel Hofman wrote:

...

Dne 03. 01. 22 v 10:10 Jaroslav Kysela napsal(a):

...

On 03. 01. 22 9:22, Pavel Hofman wrote:

...

Dne 23. 12. 21 v 9:18 Pavel Hofman napsal(a): > Hi Takashi, > > I am working on stopping alsa streams of audio USB > gadget when USB host > stops capture/playback/USB cable unplugged. > > For capture I used code from AK4114 SPDIF receiver > https://elixir.bootlin.com/linux/latest/source/sound/i2c/other/ak4114.c#L590: > > > > > static void stop_substream(struct uac_rtd_params *prm) > { >        unsigned long _flags; >        struct snd_pcm_substream *substream; > >        substream = prm->ss; >        if (substream) { >            snd_pcm_stream_lock_irqsave(substream, _flags); >            if (snd_pcm_running(substream)) >                // TODO - correct handling for playback substream? >                snd_pcm_stop(substream, SNDRV_PCM_STATE_DRAINING); >            snd_pcm_stream_unlock_irqrestore(substream, _flags); >        } > } > > For setup I found calling snd_pcm_stop(substream, > SNDRV_PCM_STATE_SETUP) > (https://elixir.bootlin.com/linux/latest/source/drivers/staging/vc04_services...) > > >     Or for both capture and playback using > SNDRV_PCM_STATE_DISCONNECTED > (https://elixir.bootlin.com/linux/latest/source/sound/core/pcm.c#L1103). > > > Or perhaps using snd_pcm_dev_disconnect(dev) or > snd_pcm_drop(substream)? > > Please what is the recommended way? >

Please can I ask for expert view on this issue? E.g. in SoX stopping the stream with SNDRV_PCM_STATE_SETUP/SNDRV_PCM_STATE_DRAINING does not stop the application, while with SNDRV_PCM_STATE_DISCONNECTED SoX exits with non-recoverable status. I am considering implementing both methods and letting users choose their suitable snd_pcm_stop operation (none (default)/SETUP-DRAINING/DISCONNECTED) for the two events (host playback/capture stop, cable disconnection) with a configfs param. Would this make sense?

The disconnection state is unrecoverable. It's expected that the device will be freed and cannot be reused.

If you expect to keep the PCM device, we should probably introduce a new function which puts the device to the SNDRV_PCM_STATE_OPEN state. In this state, all I/O routines will return -EBADFD for the applications, so they should close or re-initialize the PCM device completely.

https://elixir.bootlin.com/linux/latest/source/sound/core/pcm_native.c#L794

The fact is that after closing the USB host can re-open the device with different samplerate (and perhaps later on with different channels count/sample size). That would hint at the need to re-initialize the gadget side before opening  anyway.

As of keeping the device - it's likely some use cases would prefer keeping the device, to minimize the operations needed to react to the host-side playback/capture start.

A function you describe would make sense for this. IMO from the gadget POW there is no difference  between the host stopping playback/capture and cable disconnection, in both cases the data stream is stopped and next stream can have entirely different parameters. Maybe the gadget configfs parameter could only toggle between no action (i.e. current situation) and the new alsa function stopping the stream.

Jaroslav, please can you draft such a function? Perhaps both changes could make it to 5.17.

(Sorry for the delayed response, as I've been on vacation and now catching up the huge pile of backlogs...)

About the change to keep PCM OPEN state: I'm afraid that the disconnection in the host side may happen at any time, and keeping the state OPEN would confuse the things if the host is indeed unrecoverable.

I don't think so. The SNDRV_PCM_IOCTL_HW_PARAMS must be issued by the application (in the PCM_OPEN state) and if the USB bus connection is no longer active, it may fail. We can distinguish between host -> device disconnection and device -> host one. It is not really a similar thing.

I think that the idea was to avoid to re-build the whole card / device structure for the fixed device allocation.

Pavel, if the USB host is not connected to the gadget, where the playback PCM device fails now ? Is the PCM device created or not ?

The gaudio PCM device is created when the gadget function is activated (module loaded), regardless whether the USB host is actually connected. The playback/capture fails after the blocking read/write times out. The data delivery/consumption method is simply not called when no usb requests get completed https://elixir.bootlin.com/linux/latest/source/drivers/usb/gadget/function/u... .

The current code does basically nothing to the alsa pcm stream at capture/playback start/stop by the host (called when altsetting changes in the gadget) https://elixir.bootlin.com/linux/latest/source/drivers/usb/gadget/function/u... https://elixir.bootlin.com/linux/latest/source/drivers/usb/gadget/function/u...

Thinking about it, I think the current behaviour is probably correct.

It's not 100% possible to detect when the host stops data transfer - we can detect two scenarios:

- Cable disconnected - Interface alt 0 selected

but it's equally possible to just leave the device configured as it was and stop sending data.

While resetting state may be necessary when the cable is disconnected, if the host is just stopping and restarting the stream then I don't see why the gadget application should have to reconfigure the PCM device.

It's clearly useful to have some indication of host state, but I'm not at all convinced the PCM state is the best way to provide that.

John

Dne 04. 01. 22 v 17:21 John Keeping napsal(a):

On Tue, Jan 04, 2022 at 03:57:09PM +0000, John Keeping wrote:

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On Mon, Jan 03, 2022 at 01:54:13PM +0100, Pavel Hofman wrote:

...

Dne 03. 01. 22 v 13:28 Jaroslav Kysela napsal(a):

...

On 03. 01. 22 13:15, Takashi Iwai wrote:

...

On Mon, 03 Jan 2022 12:32:53 +0100, Pavel Hofman wrote:

...

Dne 03. 01. 22 v 10:10 Jaroslav Kysela napsal(a): > On 03. 01. 22 9:22, Pavel Hofman wrote: >> >> Dne 23. 12. 21 v 9:18 Pavel Hofman napsal(a): >>> Hi Takashi, >>> >>> I am working on stopping alsa streams of audio USB >>> gadget when USB host >>> stops capture/playback/USB cable unplugged. >>> >>> For capture I used code from AK4114 SPDIF receiver >>> https://elixir.bootlin.com/linux/latest/source/sound/i2c/other/ak4114.c#L590: >>> >>> >>> >>> >>> static void stop_substream(struct uac_rtd_params *prm) >>> { >>>        unsigned long _flags; >>>        struct snd_pcm_substream *substream; >>> >>>        substream = prm->ss; >>>        if (substream) { >>>            snd_pcm_stream_lock_irqsave(substream, _flags); >>>            if (snd_pcm_running(substream)) >>>                // TODO - correct handling for playback substream? >>>                snd_pcm_stop(substream, SNDRV_PCM_STATE_DRAINING); >>>            snd_pcm_stream_unlock_irqrestore(substream, _flags); >>>        } >>> } >>> >>> For setup I found calling snd_pcm_stop(substream, >>> SNDRV_PCM_STATE_SETUP) >>> (https://elixir.bootlin.com/linux/latest/source/drivers/staging/vc04_services...) >>> >>> >>>     Or for both capture and playback using >>> SNDRV_PCM_STATE_DISCONNECTED >>> (https://elixir.bootlin.com/linux/latest/source/sound/core/pcm.c#L1103). >>> >>> >>> Or perhaps using snd_pcm_dev_disconnect(dev) or >>> snd_pcm_drop(substream)? >>> >>> Please what is the recommended way? >>> >> >> Please can I ask for expert view on this issue? E.g. in >> SoX stopping the >> stream with >> SNDRV_PCM_STATE_SETUP/SNDRV_PCM_STATE_DRAINING does not >> stop >> the application, while with SNDRV_PCM_STATE_DISCONNECTED >> SoX exits with >> non-recoverable status. I am considering implementing both methods and >> letting users choose their suitable snd_pcm_stop operation (none >> (default)/SETUP-DRAINING/DISCONNECTED) for the two events (host >> playback/capture stop, cable disconnection) with a >> configfs param. Would >> this make sense? > > The disconnection state is unrecoverable. It's expected that the > device will be freed and cannot be reused. > > If you expect to keep the PCM device, we should probably introduce a > new function which puts the device to the SNDRV_PCM_STATE_OPEN > state. In this state, all I/O routines will return -EBADFD for the > applications, so they should close or re-initialize the PCM device > completely. > > https://elixir.bootlin.com/linux/latest/source/sound/core/pcm_native.c#L794 > >

The fact is that after closing the USB host can re-open the device with different samplerate (and perhaps later on with different channels count/sample size). That would hint at the need to re-initialize the gadget side before opening  anyway.

As of keeping the device - it's likely some use cases would prefer keeping the device, to minimize the operations needed to react to the host-side playback/capture start.

A function you describe would make sense for this. IMO from the gadget POW there is no difference  between the host stopping playback/capture and cable disconnection, in both cases the data stream is stopped and next stream can have entirely different parameters. Maybe the gadget configfs parameter could only toggle between no action (i.e. current situation) and the new alsa function stopping the stream.

Jaroslav, please can you draft such a function? Perhaps both changes could make it to 5.17.

(Sorry for the delayed response, as I've been on vacation and now catching up the huge pile of backlogs...)

About the change to keep PCM OPEN state: I'm afraid that the disconnection in the host side may happen at any time, and keeping the state OPEN would confuse the things if the host is indeed unrecoverable.

I don't think so. The SNDRV_PCM_IOCTL_HW_PARAMS must be issued by the application (in the PCM_OPEN state) and if the USB bus connection is no longer active, it may fail. We can distinguish between host -> device disconnection and device -> host one. It is not really a similar thing.

I think that the idea was to avoid to re-build the whole card / device structure for the fixed device allocation.

Pavel, if the USB host is not connected to the gadget, where the playback PCM device fails now ? Is the PCM device created or not ?

The gaudio PCM device is created when the gadget function is activated (module loaded), regardless whether the USB host is actually connected. The playback/capture fails after the blocking read/write times out. The data delivery/consumption method is simply not called when no usb requests get completed https://elixir.bootlin.com/linux/latest/source/drivers/usb/gadget/function/u... .

The current code does basically nothing to the alsa pcm stream at capture/playback start/stop by the host (called when altsetting changes in the gadget) https://elixir.bootlin.com/linux/latest/source/drivers/usb/gadget/function/u... https://elixir.bootlin.com/linux/latest/source/drivers/usb/gadget/function/u...

Thinking about it, I think the current behaviour is probably correct.

It's not 100% possible to detect when the host stops data transfer - we can detect two scenarios:

• Cable disconnected
• Interface alt 0 selected

but it's equally possible to just leave the device configured as it was and stop sending data.

While resetting state may be necessary when the cable is disconnected, if the host is just stopping and restarting the stream then I don't see why the gadget application should have to reconfigure the PCM device.

It's clearly useful to have some indication of host state, but I'm not at all convinced the PCM state is the best way to provide that.

D'oh, I totally forgot about the case of changing sample rate here, which is the new feature in Pavel's proposed patches.

I still think the existing behaviour is correct when the sample rate hasn't changed. But if the sample rate changes, maybe we have to force the gadget application to reconfigure the PCM as the available sample rate is now different. And I guess changing the PCM state is necessary in that case.

But I'd really like to avoid it for a gadget with only one available sample rate (and ideally in the case where a gadget supporting multiple sample rates is stopped and re-started at the same rate).

IMO there are several points:

1) Breaking the blocking wait in alsa read/write as soon as the gadget learns no data will arrive/be consumed by the host (altsetting to zero, cable disconnected). For me this is important as I do not want the app thread to be stuck waiting, unable to exit and especially to restart quickly (with new samplerate config).

2) Deciding what PCM state to set within the PCM stop call. For the single-samplerate config we know the next open will use the same rate, therefore the less "intrusive" status/call can be used. But for multiple samplerates I do not know what samplerate the host will use for the next opening. Also we should consider that future patches can add multiple altsettings with different channel counts (and also corresponding different samplerate sets to fit the bandwidth).

3) Compatibility with the existing behaviour for existing installations. I think the feature requires a dedicated configfs param. The param should at least disallow the PCM stop (as default). Perhaps a bitmask could be used to configure USB host stop (altsetting -> zero) and cable disconnection events separately. Maybe we could assign multiple bits to each event to allow configuring more reactions to each event (0000=default - no PCM stop, 0001 - no-reconfig PCM status, 0010 - reconfig PCM status, 0011 - DISCONNECTED status, etc.)

That way different requirements could be catered for.

I can prepare all currently-relevant patches should they be found heading in the right direction.

Thanks a lot,

Pavel.