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Date:   Thu, 15 Jun 2023 18:11:36 +0200
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>
Cc:     "Jacky Huang" <ychuang570808@gmail.com>,
        "Rob Herring" <robh+dt@kernel.org>,
        krzysztof.kozlowski+dt@linaro.org, "Lee Jones" <lee@kernel.org>,
        "Michael Turquette" <mturquette@baylibre.com>,
        "Stephen Boyd" <sboyd@kernel.org>,
        "Philipp Zabel" <p.zabel@pengutronix.de>,
        "Jiri Slaby" <jirislaby@kernel.org>,
        "Tomer Maimon" <tmaimon77@gmail.com>,
        "Catalin Marinas" <catalin.marinas@arm.com>,
        "Will Deacon" <will@kernel.org>, devicetree@vger.kernel.org,
        linux-clk@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-serial@vger.kernel.org,
        soc@kernel.org, schung@nuvoton.com, mjchen@nuvoton.com,
        "Jacky Huang" <ychuang3@nuvoton.com>
Subject: Re: [PATCH v14 1/1] tty: serial: Add Nuvoton ma35d1 serial driver support
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On Thu, Jun 15, 2023, at 17:00, Greg Kroah-Hartman wrote:
> On Thu, Jun 15, 2023 at 04:01:55PM +0200, Arnd Bergmann wrote:
>> Since the number is exported to userspace in serial_struct,
>> it might be better to pick a new constant such as
>> 
>> #define PORT_SERIAL_GENERIC (-1)
>> 
>> in order to be less ambiguous. It's a signed integer, so -1
>> would work here this would clearly be a special value, or
>> another option might be to use 255 as something that is
>> slightly less special but still recognizable as something
>> that may have a special meaning.
>
> A new constant would be good, 255 is nice, and then we can move everyone
> to use it unless they can specifically show a reason why it will not
> work for them.
>
> I think originally this was used to do device-specific ioctls, right?
> That shouldn't be happening anymore, hopefully...

The only thing I could find is that you can use TIOCSSERIAL to set
the type between the supported types within a driver, which changes
the behavior in some cases, e.g. the exact size and layout of the
register file or its capabilities.

We may need a proper audit of TIOCSSERIAL anyway, I suspect there
are worse things you can do with other settings.

       Arnd