linux-kernel - Re: [PATCH v13 50/53] ALSA: usb-audio: Allow for rediscovery of connected USB SND devices

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Date: Tue, 06 Feb 2024 14:00:19 +0100
From: Takashi Iwai <tiwai@...e.de>
To: Amadeusz Sławiński
 <amadeuszx.slawinski@...ux.intel.com>
Cc: Wesley Cheng <quic_wcheng@...cinc.com>,
	srinivas.kandagatla@...aro.org,
	mathias.nyman@...el.com,
	perex@...ex.cz,
	conor+dt@...nel.org,
	corbet@....net,
	lgirdwood@...il.com,
	andersson@...nel.org,
	krzysztof.kozlowski+dt@...aro.org,
	gregkh@...uxfoundation.org,
	Thinh.Nguyen@...opsys.com,
	broonie@...nel.org,
	bgoswami@...cinc.com,
	tiwai@...e.com,
	robh+dt@...nel.org,
	konrad.dybcio@...aro.org,
	linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org,
	linux-sound@...r.kernel.org,
	linux-usb@...r.kernel.org,
	linux-arm-msm@...r.kernel.org,
	linux-doc@...r.kernel.org,
	alsa-devel@...a-project.org
Subject: Re: [PATCH v13 50/53] ALSA: usb-audio: Allow for rediscovery of connected USB SND devices

On Mon, 05 Feb 2024 10:01:03 +0100,
Amadeusz Sławiński wrote:
> 
> On 2/3/2024 3:36 AM, 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@...cinc.com>
> > ---
> >   sound/usb/card.c                  | 19 +++++++++++++++++++
> >   sound/usb/card.h                  |  2 ++
> >   sound/usb/qcom/qc_audio_offload.c |  2 ++
> >   3 files changed, 23 insertions(+)
> > 
> > diff --git a/sound/usb/card.c b/sound/usb/card.c
> > index 11b827b7a2a5..995b2df676ab 100644
> > --- a/sound/usb/card.c
> > +++ b/sound/usb/card.c
> > @@ -202,6 +202,25 @@ 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(&register_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]);
> 
> if inside if, it can just be && or maybe move callback check before
> mutex lock and just return early if it is not present?

The callback check must be inside mutex; otherwise you'll get a race
about the platform_ops registration.


thanks,

Takashi