Re: [PATCH] USB: serial: option: set driver_info for SIM5218 and compatibles

[Dan Williams <dcbw@redhat.com>]

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@kernel.org> writes:
> > > 
> > > > On Tue, Mar 19, 2019 at 10:54:00AM +0000, Måns Rullgård wrote:
> > > > > Johan Hovold <johan@kernel.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.

Dan