linux-kernel - Re: [PATCH 2/2] usb: serial: xr

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Message-ID: <20200519085703.GB27787@localhost>
Date:   Tue, 19 May 2020 10:57:03 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Manivannan Sadhasivam <mani@...nel.org>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Johan Hovold <johan@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        linux-usb <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        patong.mxl@...il.com,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH 2/2] usb: serial: xr_serial: Add gpiochip support

On Wed, Apr 29, 2020 at 06:19:18PM +0530, Manivannan Sadhasivam wrote:

> On Wed, Apr 29, 2020 at 02:12:24PM +0200, Linus Walleij wrote:
> > On Tue, Apr 28, 2020 at 9:57 PM <mani@...nel.org> wrote:
> > 
> > > From: Manivannan Sadhasivam <mani@...nel.org>
> > >
> > > Add gpiochip support for Maxlinear/Exar USB to serial converter
> > > for controlling the available gpios.
> > >
> > > Cc: Linus Walleij <linus.walleij@...aro.org>
> > > Cc: linux-gpio@...r.kernel.org
> > > Signed-off-by: Manivannan Sadhasivam <mani@...nel.org>

> > I would change this:
> > 
> >    port_priv->gc.label = "xr_gpios";
> > 
> > to something that is device-unique, like "xr-gpios-<serial number>"
> > which makes it easy to locate the GPIOs on a specific serial converter
> > for lab use. However the USB serial maintainers know better what
> > to use here. Whatever makes a USB-to-serial unique from a TTY
> > point of view is probably fine with me too.
> > 
> > My idea is that people might want to know which USB cable
> > this is sitting on, so I have this USB cable and from this label
> > I can always figure out which GPIO device it is.

I think we've had this discussion before. First, not every device has a
unique serial number. Second, we already have a universal way of
distinguishing devices namely by using the bus topology. That's
available through sysfs and shouldn't have to be be re-encoded by every
driver in the gpiochip name.

> Sounds reasonable. I can postfix the PID as below:
>
> port_priv->gc.label = devm_kasprintf(port->dev, GFP_KERNEL, "XR%04x",
>                                      port_priv->idProduct);
> 
> So this will become, "XR1410".

So this doesn't really buy us anything; what if you have two of these
devices?

Johan