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Date:   Tue, 19 May 2020 11:08:53 +0200
From:   Johan Hovold <johan@kernel.org>
To:     Manivannan Sadhasivam <mani@kernel.org>
Cc:     gregkh@linuxfoundation.org, johan@kernel.org,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        patong.mxl@gmail.com, Linus Walleij <linus.walleij@linaro.org>,
        linux-gpio@vger.kernel.org
Subject: Re: [PATCH 2/2] usb: serial: xr_serial: Add gpiochip support
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References: <20200428195651.6793-1-mani@kernel.org>
 <20200428195651.6793-3-mani@kernel.org>
 <20200429174727.GF6443@Mani-XPS-13-9360>
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On Wed, Apr 29, 2020 at 11:17:27PM +0530, Manivannan Sadhasivam wrote:

> On Wed, Apr 29, 2020 at 01:26:51AM +0530, mani@kernel.org wrote:
> > From: Manivannan Sadhasivam <mani@kernel.org>
> > 
> > Add gpiochip support for Maxlinear/Exar USB to serial converter
> > for controlling the available gpios.
> > 
> > Cc: Linus Walleij <linus.walleij@linaro.org>
> > Cc: linux-gpio@vger.kernel.org
> > Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>

> >  static int xr_port_probe(struct usb_serial_port *port)
> >  {
> >  	struct usb_serial *serial = port->serial;
> > @@ -495,13 +678,14 @@ static int xr_port_probe(struct usb_serial_port *port)
> >  
> >  	usb_set_serial_port_data(port, port_priv);
> >  
> > -	return 0;
> > +	return xr_gpio_init(port);
> 
> Just realised that the gpiochip is registered for 2 interfaces exposed by
> this chip. This is due to the fact that this chip presents CDC-ACM model,
> so there are 2 interfaces (interrupt and bulk IN/OUT).
> 
> We shouldn't need gpiochip for interface 0. So what is the recommended way
> to filter that?

Your driver should only bind to the data interface, but also claim the
control interface (i.e. the reverse of what cdc-acm is doing).

This CDC model doesn't really fit the assumptions of usb-serial core,
but it might be doable. Try returning 1 from the attach callback for the
control interface so that core claims it but doesn't register a tty
device.

Johan