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Date:   Tue, 31 Oct 2023 14:51:45 +0100
Subject: Re: [PATCH 6/6] tty: serial: amba-pl011: Parse bits option as 5, 6,
 7 or 8 in _get_options
Cc:     "Hugo Villeneuve" <hugo@hugovil.com>,
        "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
        "Jiri Slaby" <jirislaby@kernel.org>,
        <linux-kernel@vger.kernel.org>, <linux-serial@vger.kernel.org>,
        "Linus Walleij" <linus.walleij@linaro.org>,
        "Gregory CLEMENT" <gregory.clement@bootlin.com>,
        "Alexandre Belloni" <alexandre.belloni@bootlin.com>,
        "Thomas Petazzoni" <thomas.petazzoni@bootlin.com>,
        "Vladimir Kondratiev" <vladimir.kondratiev@mobileye.com>,
        "Tawfik Bayouk" <tawfik.bayouk@mobileye.com>
To:     "Russell King (Oracle)" <linux@armlinux.org.uk>
From:   =?utf-8?q?Th=C3=A9o_Lebrun?= <theo.lebrun@bootlin.com>
Message-Id: <CWMNJ47MO8E6.7CXJRZ181PXJ@tleb-bootlin-xps13-01>
References: <20231026-mbly-uart-v1-0-9258eea297d3@bootlin.com>
 <20231026-mbly-uart-v1-6-9258eea297d3@bootlin.com>
 <20231026105329.0ee9603563202bd2157a7d27@hugovil.com>
 <CWMITJ9VX9IP.1WPQCX981VRDE@tleb-bootlin-xps13-01>
 <ZUDS5UpWlo+DUZc4@shell.armlinux.org.uk>
 <CWMKPFZ9LOVD.2756QU9AP6U3W@tleb-bootlin-xps13-01>
 <ZUDjhpQKgUgqVeBh@shell.armlinux.org.uk>
In-Reply-To: <ZUDjhpQKgUgqVeBh@shell.armlinux.org.uk>
Precedence: bulk

On Tue Oct 31, 2023 at 12:22 PM CET, Russell King (Oracle) wrote:
> On Tue, Oct 31, 2023 at 12:04:11PM +0100, Th=C3=A9o Lebrun wrote:
> > On Tue Oct 31, 2023 at 11:11 AM CET, Russell King (Oracle) wrote:
> > > There is no point in supporting 5 or 6 bits for console usage. Think
> > > about it. What values are going to be sent over the console? It'll be
> > > ASCII, which requires at _least_ 7-bit. 6-bit would turn alpha
> > > characters into control characters, punctuation and numbers. 5-bit
> > > would be all control characters.
> > >
> > > So there's no point trying to do anything with 5 or 6 bits per byte,
> > > and I decided we might as well take that as an error (or maybe a
> > > case that the hardware has not been setup) and default to 8 bits per
> > > byte.
> >=20
> > I see your point. Two things come to mind:
> >=20
> >  - I added this parsing of 5/6 bits to be symmetrical with
> >    pl011_set_termios that handles 5/6 properly. Should pl011_set_termio=
s
> >    be modified then?
>
> Why should it? Note that I said above about _console_ usage which is
> what you were referring to - the early code that sets up the console
> by either reading the current settings (so that we can transparently
> use the UART when its handed over already setup by a boot loader).
>
> This is completely different to what happens once the kernel is running.
> Userspace might very well have a reason to set 5 or 6 bits if it wants
> to communicate with a device that uses those sizes.
>
> However, such a device won't be a console for the reasons I outlined
> above (it will truncate the ASCII characters turning console messages
> into garbage.)

I'm not sure I get it. (1) We assume it is a console so it's ASCII so no
reason to set to 5 or 6 bits per word. But (2) there might be a reason
to set the UART to 5 or 6 bits, the userspace decides.

How do the two interact? Say we boot to Linux, userspace configures to 6
bits because reasons and we reset. At second probe we see a config of 6
bits per word but assume that can't be logical, even though it is.

What makes us suppose at probe that it must be a console?

I won't die on a hill for this topic; we'll go the way you prefer!

Regards,

--
Th=C3=A9o Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com