linux-kernel - Re: [PATCH] USB: serial: option: set driver

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Message-ID: <yw1xmulqzv7k.fsf@mansr.com>
Date:   Tue, 19 Mar 2019 16:26:39 +0000
From:   Måns Rullgård <mans@...sr.com>
To:     Bjørn Mork <bjorn@...k.no>
Cc:     Dan Williams <dcbw@...hat.com>, Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: serial: option: set driver_info for SIM5218 and compatibles

Bjørn Mork <bjorn@...k.no> writes:

> Måns Rullgård <mans@...sr.com> writes:
>> Dan Williams <dcbw@...hat.com> writes:
>>
>>> TLDR; some firmware uses the DTR signal as an indicator to come out of
>>> low-power mode. Without doing so you cannot talk to the modem over any
>>> of it's ports, QMI, net, or serial.
>>
>> I must be missing something, but how does a network interface have a DTR
>> signal?
>
> It does not.  But the network function is (ab)using the Comm class USB
> control message "SetControlLineState" to signal "wake up" from the host
> to the device.  Which is perfectly fine since the USB function in
> question is vendor specific.  The vendor can define any USB control
> message to mean anything they want. Reusing a Comm class message
> probably made sense to the firmware engineer designing this.  We can
> only note and adapt.
>
> It doesn't have anything to do with DTR.

Every time you think USB can't get any worse, it does.

-- 
Måns Rullgård