linux-kernel - Re: [PATCH] USB: Add quirk for WORLDE easykey.25 MIDI keyboard

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Date:   Tue, 24 Jan 2017 09:30:51 +0100
From:   Oliver Neukum <oneukum@...e.com>
To:     Lukáš Lalinský <lukas@...gene.sk>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCH] USB: Add quirk for WORLDE easykey.25 MIDI keyboard

Am Dienstag, den 24.01.2017, 08:37 +0100 schrieb Lukáš Lalinský:
> On Tue, Jan 24, 2017 at 8:32 AM, Oliver Neukum <oneukum@...e.com>
> wrote:
> > 
> > Am Montag, den 23.01.2017, 19:36 +0100 schrieb Lukáš Lalinský:
> > > 
> > > I have uploaded both captures here -
> > > https://gist.github.com/lalinsky/83148a827d5cd43e79e377d8e1b5ed0d
> > 
> > Indeed it is does not set a configuration. Either the capture
> > is incomplete or device and host violate the standard. A device
> > may be left unconfigured.
> 
> Is this may or may not? I'm not familiar with USB, so I assumed if

You can leave a device unconfigured. In that case it stays in the
addressed state, where its power draw is strictly limited.
That is exactly what the kernel does when it is confronted with
a device that would overdraw the power budget.

> there is only one configuration and there is always one active, it
> does not need to be set explicitly because the correct one is already
> active.

No, it is not. Or rather it should not be. Can you please recheck
that you are capturing the whole exchange?

> > We need to read the descriptors even if we
> > see only one configuration to get the power budgeting right.
> 
> Aren't those in the CONFIGURATION descriptors? Reading the STRING

True.

> descriptor is probably only useful if you need to print the
> configuration details somewhere.

Also true.

> > 
> > Does the device work without any .ini file?
> 
> Yes. It's a standard MIDI device, no configuration is required.

So Windows by default does not read string descriptors. That is worth
a thought, but we need to check. Yet I am not sure how useful that is.
The first lsusb would crash the device. We'd need a quirk anyway.

	Regards
		Oliver