Re: [PATCH 3/3] usb: cp210x: Add ioctl for GPIO support

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, May 02, 2012 at 09:49:01PM +0100, Alan Cox wrote:
> > Actually, why can't you use the GPIO subsystem for something like this?
> > Can't you export your device as both a usb-serial device and a gpio
> > device and have things work properly that way?
> 
> You still need the ioctls even then in order to discover the gpio
> numbers

What discovery?  The device knows what gpio values it has in it, and
should be able to register those with the gpio subsystem.

> and having done that youi've got potential races with unload
> when you try and open them. You've also got permissions considerations
> and synchronization between gpio and data problems.

That can be handled in the driver itself, if it really is a problem (the
existing patch sure didn't handle any of that at all, so I'm guessing
either it wasn't considered, or it isn't a problem.)

> It's not a good way to go. It might make sense in some platforms to
> expose them as both but its not a good general model.

Why isn't the gpio subsystem a good general model?  I thought that is
what it was created to solve?

> I'm currently favouring adding some 'additional control line' bits to
> termiox.

Yes, but that's only good for usb-serial devices that also have gpio
pins on the controller side.  Which seems pretty limited to me.

If I have a userspace program, and I want to use GPIO, I shouldn't have
to care/know that the pins are really on the end of a USB->serial
bridge, or somewhere hanging off of a SOC, the same userspace api should
"just work", right?

Or am I missing something really obvious here?

thanks,

greg k-h