Hi Takashi,
On 2/6/2024 11:05 PM, Takashi Iwai wrote:
On Wed, 07 Feb 2024 01:08:00 +0100, Wesley Cheng wrote:
Hi Takashi,
On 2/6/2024 5:07 AM, Takashi Iwai wrote:
On Sat, 03 Feb 2024 03:36:27 +0100, Wesley Cheng wrote:
With USB audio offloading, an audio session is started from the ASoC platform sound card and PCM devices. Likewise, the USB SND path is still readily available for use, in case the non-offload path is desired. In order to prevent the two entities from attempting to use the USB bus, introduce a flag that determines when either paths are in use.
If a PCM device is already in use, the check will return an error to userspace notifying that the stream is currently busy. This ensures that only one path is using the USB substream.
Signed-off-by: Wesley Cheng quic_wcheng@quicinc.com
Hm, I'm not sure whether it's safe to hold chip->mutex there for the long code path. It even kicks off the auto-resume, which may call various functions at resuming, and some of them may re-hold chip->mutex.
That's a good point.
If it's only about the open flag, protect only the flag access with the mutex, not covering the all open function. At least the re-entry can be avoided by that.
Sure, let me re-order the check/assignment and the mutex locking. Since this is now checked here in USB PCM and the QC offload driver, we want to make sure that if there was some application attempting to open both at the same time, we prevent any possible races.
I think the best way to address this would be something like:
static int snd_usb_pcm_open(struct snd_pcm_substream *substream) { ... mutex_lock(&chip->mutex); if (subs->opened) { mutex_unlock(&chip->mutex); return -EBUSY; } subs->opened = 1; mutex_unlock(&chip->mutex);
//Execute bulk of PCM open routine ... return 0;
// If any errors are seen, unwind err_resume: snd_usb_autosuspend(subs->stream->chip); err_open: mutex_lock(&chip->mutex); subs->opened = 0; mutex_unlock(&chip->mutex);
return ret; }
Set the opened flag first, so that if QC offload checks it, it can exit early and vice versa. Otherwise, if we set the opened flag at the same position as the previous patch, we may be calling the other routines in parallel to the QC offload enable stream routine. The only thing with this patch is that we'd need some error handling unwinding.
The above is what I had in mind.
But, thinking on this again, you might be able to get the same result by using the ALSA PCM core substream open_mutex and hw_opened flag. This is already held and set at snd_pcm_core() (the hw_opened flag is set after open callback, though). The offload driver can use those instead of the own lock and flag, too, although it's not really well-mannered behavior (hence you need proper comments).
I think I had looked into this as well previously, and it was difficult to achieve, because from the USB offloading perspective, we don't ever call: snd_usb_pcm_open()
This is actually where we populate the pcm_substream parameter within struct snd_usb_substream based on when userspace opens the USB SND PCM device (which is not the case for offloading). So the offload driver doesn't have a way to fetch the struct snd_pcm that is allocated to the PCM device created by the USB SND card.
Thanks Wesley Cheng