Re: [PATCH v2] ALSA: usb-audio: Support multiple control interfaces

[Takashi Iwai <tiwai@xxxxxxx>]

On Mon, 12 Aug 2024 02:29:56 +0200,
Karol Kosik wrote:
> 
> Registering Numark Party Mix II fails with error 'bogus bTerminalLink 1'.
> The problem stems from the driver not being able to find input/output
> terminals required to configure audio streaming. The information about
> those terminals is stored in AudioControl Interface. Numark device
> contains 2 AudioControl Interfaces and the driver checks only one of them.
> 
> According to the USB standard, a device can have multiple audio functions,
> each represented by Audio Interface Collection. Every audio function is
> considered to be closed box and will contain unique AudioControl Interface
> and zero or more AudioStreaming and MIDIStreaming Interfaces.
> 
> The Numark device adheres to the standard and defines two audio functions:
> - MIDIStreaming function
> - AudioStreaming function
> It starts with MIDI function, followed by the audio function. The driver
> saves the first AudioControl Interface in `snd_usb_audio` structure
> associated with the entire device. It then attempts to use this interface
> to query for terminals and clocks. However, this fails because the correct
> information is stored in the second AudioControl Interface, defined in the
> second Audio Interface Collection.
> 
> This patch introduces a structure holding association between each
> MIDI/Audio Interface and its corresponding AudioControl Interface,
> instead of relying on AudioControl Interface defined for the entire
> device. This structure is populated during usb probing phase and leveraged
> later when querying for terminals and when sending USB requests.
> 
> Alternative solutions considered include:
> - defining a quirk for Numark where the order of interface is manually
> changed, or terminals are hardcoded in the driver. This solution would
> have fixed only this model, though it seems that device is USB compliant,
> and it also seems that other devices from this company may be affected.
> What's more, it looks like products from other manufacturers have similar
> problems, i.e. Rane One DJ console
> - keeping a list of all AudioControl Interfaces and querying all of them
> to find required information. That would have solved my problem and have
> low probability of breaking other devices, as we would always start with
> the same logic of querying first AudioControl Interface. This solution
> would not have followed the standard though.
> 
> This patch preserves the `snd_usb_audio.ctrl_intf` variable, which holds
> the first AudioControl Interface, and uses it as a fallback when some
> interfaces are not parsed correctly and lack an associated AudioControl
> Interface, i.e., when configured via quirks.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217865
> 
> Signed-off-by: Karol Kosik <k.kosik@xxxxxxxxxxx>

Applied now to for-next branch.  Thanks!


Takashi