linux-kernel - Re: [PATCH] USB: serial: ftdi_sio: add ftdi serial to gpiochip label

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Message-ID: <4193075.XuvPJYOev2@dl3541lx>
Date: Mon, 01 Jan 2024 18:00:22 -0800
From: Stan Bertrand <stanislasbertrand@...il.com>
To: Stan Bertrand <stanislasbertrand@...il.com>, Johan Hovold <johan@...nel.org>
Cc: linux-usb@...r.kernel.org, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: serial: ftdi_sio: add ftdi serial to gpiochip label

On Friday, December 15, 2023 2:10:15 AM PST Johan Hovold wrote:
> On Mon, Oct 02, 2023 at 05:11:35PM -0700, Stan Bertrand wrote:
> > From: Stanislas Bertrand <stanislasbertrand@...il.com>
> > 
> > Use ftdi serial number on gpiochip label.
> > Allows to interface with gpiod utils using the serial number:
> > 
> > $ gpiodetect
> > gpiochip5 [ftdi-cbus-FTRelay2] (4 lines)
> > gpiochip6 [ftdi-cbus] (4 lines)
> > gpiochip7 [ftdi-cbus-A106TPEC] (4 lines)
> > 
> > $ gpioget ftdi-cbus-FTRelay2 2
> > 0
> 
> I don't think this is a good idea, for example, as not all devices have
> a unique serial string.

The goal is to identify the gpiochip corresponding to a known FTDI device.
If serials are not unique, it can still differentiate others.
A device serial can be set (FT Prog, Python lib, ...) for direct of access.

> Looks like the naming of gpiochips are all over the place, and ideally
> this should not have been something that was left up to individual
> driver to decide.
> 
> I see several drivers using the name of the corresponding platform
> device as label, which works in most cases, but not always either. The
> only unique and always available identifier is the gpiochip's place in
> the device tree itself.
> 
> For USB, we already encode the bus topology in the USB device names
> (e.g. 1-11.5.1) and we could possibly consider using that. But we
> already have USB serial devices with multiple GPIO chips so also that
> would require some further thought (e.g. using the interface name
> instead).

The aim is identification while being platform agnostic, device tree or x86.
The FTDI serial allows device identification regardless of the system topology.

> Johan

Thanks,
Stan