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

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Date: Mon, 5 Feb 2024 13:27:52 -0800
From: Wesley Cheng <quic_wcheng@...cinc.com>
To: Amadeusz Sławiński
	<amadeuszx.slawinski@...ux.intel.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>
CC: <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

Hi Amadeusz,

On 2/5/2024 1:01 AM, 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?
> 

Thanks for pointing that out.  Makes sense, and will update it.

Thanks
Wesley Cheng