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

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Date:   Tue, 19 Mar 2019 14:35:18 +0000
From:   Måns Rullgård <mans@...sr.com>
To:     Dan Williams <dcbw@...hat.com>
Cc:     Johan Hovold <johan@...nel.org>,
        Bjørn Mork <bjorn@...k.no>,
        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

Dan Williams <dcbw@...hat.com> writes:

> On Tue, 2019-03-19 at 13:43 +0100, Johan Hovold wrote:
>> On Tue, Mar 19, 2019 at 01:27:19PM +0100, Johan Hovold wrote:
>> > On Tue, Mar 19, 2019 at 12:25:53PM +0000, Måns Rullgård wrote:
>> > > Johan Hovold <johan@...nel.org> writes:
>> > > 
>> > > > On Tue, Mar 19, 2019 at 10:54:00AM +0000, Måns Rullgård wrote:
>> > > > > Johan Hovold <johan@...nel.org> writes:
>> > > > > > > Regardless, setting the NCTRL flag should be harmless.
>> > > > > > 
>> > > > > > Well, there are devices that depend on getting these
>> > > > > > requests, at least
>> > > > > > for the QMI interface. But we can always revert if anyone
>> > > > > > complains.
>> > > > > 
>> > > > > The QMI interface doesn't even pretend to be a uart.  The
>> > > > > other ones do,
>> > > > > but there isn't actually any real uart behind them.  For
>> > > > > instance, it
>> > > > > doesn't matter what baud rate one sets.
>> > > > 
>> > > > Sure, but some devices still require "DTR" to be set for the
>> > > > QMI
>> > > > interface, so there not being any real uart is no guarantee
>> > > > that there
>> > > > is no firmware that expects these calls.
>> > > 
>> > > Now I'm thoroughly confused.  The QMI interface has a completely
>> > > separate driver that creates a network device (if I'm reading the
>> > > code
>> > > correctly).
>> > 
>> > I was just giving an example of firmware sometimes doing unexpected
>> > things.
>> 
>> See 93725149794d ("net: qmi_wwan: MDM9x30 specific power management")
>> for some background.
>
> 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?

-- 
Måns Rullgård