linux-kernel - Re: [PATCH v2] USB: serial: cp210x: Fix GPIO in autosuspend

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Message-ID: <20190319131409.GL6124@localhost>
Date:   Tue, 19 Mar 2019 14:14:09 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Oliver Neukum <oneukum@...e.com>
Cc:     Johan Hovold <johan@...nel.org>,
        Loic Poulain <loic.poulain@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Karoly Pados <pados@...os.hu>, linux-kernel@...r.kernel.org,
        linux-usb@...r.kernel.org
Subject: Re: [PATCH v2] USB: serial: cp210x: Fix GPIO in autosuspend

On Tue, Mar 19, 2019 at 12:15:06PM +0100, Oliver Neukum wrote:
> On Di, 2019-03-19 at 11:36 +0100, Johan Hovold wrote:
> > On Mon, Feb 18, 2019 at 10:17:12AM +0100, Oliver Neukum wrote:
> > > On So, 2019-02-17 at 18:59 +0100, Karoly Pados wrote:
> > > > Current GPIO code in cp210x fails to take USB autosuspend into account,
> > > > making it practically impossible to use GPIOs with autosuspend enabled
> > > > without user configuration. Fix this like for ftdi_sio in a previous patch.
> > > > Tested on a CP2102N.
> > > your patch is looking good to me, but I am afraid there are issues.
> > > How do the GPIO lines on the device interact with USB reset and system
> > > suspend?
> > 
> > What was your concern here, Oliver?
> 
> If you have a device resetting or losing power (for reset_resume) the
> GPIO lines will revert to the default. But that change is not reported
> to user space, is it?

This driver doesn't support reset_resume() so that shouldn't be an
issue, right? We'd disconnect (deregister the gpiochip) and re-probe
instead.

> So the original patch is correct, but there are more situations rather
> than suspend which could trigger the problem.

This patch just added the missing auto-resume handling, but, yeah,
there may be devices out there for which things may get out of sync if
they lose state over suspend. I think Karoly confirmed this wasn't the
case with cp210x.

Johan