linux-kernel - Re: [alsa-devel] [PATCH v2] ALSA: usb-audio: Add support for Akai MPC Element USB MIDI controller

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Date:	Fri, 09 Jan 2015 09:48:42 +0100
From:	Clemens Ladisch <clemens@...isch.de>
To:	Paul Bonser <misterpib@...il.com>,
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.de>
CC:	alsa-devel@...a-project.org, linux-kernel@...r.kernel.org
Subject: Re: [alsa-devel] [PATCH v2] ALSA: usb-audio: Add support for Akai
	MPC Element USB MIDI controller

Paul Bonser wrote:
> On 01/08/2015 03:56 PM, Clemens Ladisch wrote:
>> Paul Bonser wrote:
>>> The Akai MPC Element incorrectly reports its bInterfaceClass as 255, but
>>> otherwise implements the USB MIDI spec correctly.
>>>
>>> This adds a quirks-table.h entry which allows the device to be
>>> recognized as a standard USB MIDI device.
>>>
>>> +++ b/sound/usb/quirks-table.h
>>> +{
>>> +	/* Akai MPC Element */
>>> +	USB_DEVICE(0x09e8, 0x0021),
>>> +	.driver_info = (unsigned long) & (const struct snd_usb_audio_quirk) {
>>> +		.ifnum = QUIRK_ANY_INTERFACE,
>>> +		.type = QUIRK_COMPOSITE,
>>> +		.data = & (const struct snd_usb_audio_quirk[]) {
>>> +			{
>>> +				.ifnum = 0,
>>> +				.type = QUIRK_IGNORE_INTERFACE
>>> +			},
>>> +			{
>>> +				.ifnum = 1,
>>> +				.type = QUIRK_MIDI_STANDARD_INTERFACE
>>> +			},
>>> +			{
>>> +				.ifnum = -1
>>> +			}
>>> +		}
>>> +	}
>>> +},
>>
>> Why a composite quirk?  Does a single quirk entry not work?
>
> A single quirk entry results in an error message in dmesg:
> "snd-usb-audio: probe of 3-2:1.0 failed with error -5"

That message is normal for unhandled interfaces.

Does it actually prevent the device from working?


Regards,
Clemens
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