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@xxxxxxxxxxx>
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
