linux-kernel - Re: [PATCH 0/2] FTDI CBUS GPIO support

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Date:	Tue, 23 Jun 2015 11:22:19 +0200
From:	Johan Hovold <johan@...nel.org>
To:	Stefan Agner <stefan@...er.ch>
Cc:	Johan Hovold <johan@...nel.org>, linus.walleij@...aro.org,
	gnurou@...il.com, grant.likely@...retlab.ca,
	gregkh@...uxfoundation.org, x-linux@...ra-silbe.de,
	hachti@...hti.de, linux-usb@...r.kernel.org,
	linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
	Johan Hovold <jhovold@...il.com>
Subject: Re: [PATCH 0/2] FTDI CBUS GPIO support

On Mon, Jun 22, 2015 at 10:11:35PM +0200, Stefan Agner wrote:
> On 2015-06-22 19:26, Johan Hovold wrote:

> > Instead, hang the gpio chip directly off the usb interface (not the
> > port), add a new config option, and keep the gpio implementation under
> > drivers/usb/serial (possibly in its own file ftdi_sio-gpio.c).
> 
> Agreed sounds like a good plan. Will try this approach in v2.
> 
> Except I don't think hanging it directly to the USB interface is the
> right thing to do.
> 
> Looking at the block diagram of FT232R or FT232H, the CBUS pins seem to
> be part of the UART/FIFO controller. And I think the dual UART FT2232D
> actually supports controlling the CBUS pins of the two UART controllers
> individually, at least the block diagram thereof suggests so.

The port is a Linux abstraction, and for FTDI we happen to have exactly
one port child device per USB interface. As I see it, the gpio
controller for the CBUS pins should be a sibling rather than a child
device to the port.

Note that we'd still have two gpio-controllers on FT2232D (one per USB
interface).

I'm aware that this requires some restructuring of the ftdi_sio-driver
(e.g. the device type and ftdi-interface number should be a feature of
the usb-serial rather than usb-serial-port device).

Thanks,
Johan
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