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

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Date:   Tue, 19 Mar 2019 13:43:58 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Måns Rullgård <mans@...sr.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

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.

Johan