linux-kernel - Re: [PATCH v4] USB: serial: ftdi_sio: implement GPIO support for FT-X devices

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Date:   Tue, 18 Sep 2018 10:27:58 +0000
From:   "Karoly Pados" <pados@...os.hu>
To:     "Johan Hovold" <johan@...nel.org>
Cc:     "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Loic Poulain" <loic.poulain@...aro.org>
Subject: Re: [PATCH v4] USB: serial: ftdi_sio: implement GPIO support for
 FT-X devices

> We'll get this into 4.20 either way, there's plenty of time. But I guess
> we could play it safe and always register four pins, and if/when we get
> more info about FT234XD we can implement registering just one pin in a
> follow up patch.
> 
> Sounds good? If so I'll just merge your v5 (registering four pins) next
> week.

Ofc, thank you.

>> Does that mean something can call into our module while this method is running?
>> If not, I'm clueless about the possible race here.
> 
> Correct, you can get gpio callbacks until the gpio chip has been
> deregistered (anything coming in after that would be a gpiolib bug).

I'm just puzzled by the fact that this can still happen even while the port 
removal code is running. It also makes me ask the question what else here can
run in parallel. Don't we need mutexes here-and-there in the ftx gpio functions too
in this case? Or does gpiolib make sure to serialize multiple pin and direction
setting calls?

Thank you,
Karoly