linux-kernel - Re: [PATCH] USB: ftdi

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Date:	Fri, 13 Jun 2014 09:25:07 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Sascha Silbe <x-linux@...ra-silbe.de>
Cc:	Johan Hovold <jhovold@...il.com>,
	Alexandre Courbot <gnurou@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] USB: ftdi_sio: add GPIO support

On Mon, Jun 9, 2014 at 3:21 PM, Sascha Silbe <x-linux@...ra-silbe.de> wrote:

> The chips can operate either in regular or in bitbang mode. Care was
> taken to prevent using GPIOs if the serial device is in use and vice
> versa.

Very interesting patch! I've seen USB-based GPIO things before
but never a dual-mode thing.

There was already a comment to move the implementation to a
separate file, which I won't repeat.

But I also want to bring the device model into question: normally
when a mother device spawns children across different subsystems
we model them as MFD devices (drivers/mfd) that instantiate
children for the different subsystems. So you could spawn a
serial and a GPIO device from a USB-based hub device there.

I do not know if that is really apropriate in this case. It seems the
device is first and foremost FTDI.

But it could still spawn a child platform device for the GPIO stuff
so that this can live as a separate driver under drivers/gpio/gpio-ftdi.c
or similar.

You could then use something like:

struct platform_device *gdev;

gdev = platform_device_alloc("gpio-ftdi", PLATFORM_DEVID_AUTO);
/* pdata contains communication cookie for callbacks etc */
ret = platform_device_add_data(gdev, pdata, sizeof(*pdata));
ret = platform_device_add(gdev);

Then we can probe that device normally in the GPIO subsystem
like any other driver, just that it needs some
<linux/usb/ftdi.h> header or similar to define the function
calls to communicate with the FTDI driver.

However Greg is device core maintainer and may have better
ideas about this!

Yours,
Linus Walleij
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