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SubjectRe: [PATCH v5 11/32] sound: usb: card: Introduce USB SND platform op callbacks
Hi Takashi,

On 9/7/2023 8:36 AM, Takashi Iwai wrote:
> On Tue, 29 Aug 2023 23:06:36 +0200,
> Wesley Cheng wrote:
>>
>> Allow for different platforms to be notified on USB SND connect/disconnect
>> seqeunces. This allows for platform USB SND modules to properly initialize
>> and populate internal structures with references to the USB SND chip
>> device.
>>
>> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
>> ---
>> sound/usb/card.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>> sound/usb/card.h | 9 +++++++++
>> 2 files changed, 54 insertions(+)
>>
>> diff --git a/sound/usb/card.c b/sound/usb/card.c
>> index 1b2edc0fd2e9..067a1e82f4bf 100644
>> --- a/sound/usb/card.c
>> +++ b/sound/usb/card.c
>> @@ -118,6 +118,34 @@ MODULE_PARM_DESC(skip_validation, "Skip unit descriptor validation (default: no)
>> static DEFINE_MUTEX(register_mutex);
>> static struct snd_usb_audio *usb_chip[SNDRV_CARDS];
>> static struct usb_driver usb_audio_driver;
>> +static struct snd_usb_platform_ops *platform_ops;
>> +
>> +int snd_usb_register_platform_ops(struct snd_usb_platform_ops *ops)
>> +{
>> + int ret;
>> +
>> + mutex_lock(&register_mutex);
>> + if (platform_ops) {
>> + ret = -EEXIST;
>> + goto out;
>> + }
>> +
>> + platform_ops = ops;
>> +out:
>> + mutex_unlock(&register_mutex);
>> + return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(snd_usb_register_platform_ops);
>
> For adding this kind of API, please give the proper comment.
> Especially this API is special and need a caution, to mention that it
> can be used only for a single instance.
>
> Also, it should be mentioned that all callbacks are exclusive under
> the global register_mutex.
>

Thanks for taking the time to review. Sure, I'll add some comments in
these new APIs to document what they are used for and how they are
protected and limited.

>> @@ -910,7 +938,11 @@ static int usb_audio_probe(struct usb_interface *intf,
>> chip->num_interfaces++;
>> usb_set_intfdata(intf, chip);
>> atomic_dec(&chip->active);
>> +
>> + if (platform_ops && platform_ops->connect_cb)
>> + platform_ops->connect_cb(chip);
>> mutex_unlock(&register_mutex);
>
> One uncertain thing is the argument for connect_cb and disconnect_cb.
> Those take snd_usb_audio object, but the callback gets called per
> interface at each probe and disconnect. How does the callee handle
> multiple calls?

I guess it should depend on how the platform driver wants to handle it?
I haven't run into a device with multiple UAC interfaces before, so
I'll need to mimic this configuration on a device, so I can see how it
exposes itself.

Will investigate this a bit more on my end and come back with my findings.

>
> Last but not least, the patch subject should be with "ALSA:" prefix,
> and in this case, at best "ALSA: usb-audio: xxx".
>
>

Got it, thanks!

Thanks
Wesley Cheng

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