linux-kernel - Re: [PATCH 6/6] tty: serial: amba-pl011: Parse bits option as 5, 6, 7 or 8 in _get

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Message-Id: <CWMNJ47MO8E6.7CXJRZ181PXJ@tleb-bootlin-xps13-01>
Date:   Tue, 31 Oct 2023 14:51:45 +0100
From:   Théo Lebrun <theo.lebrun@...tlin.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     "Hugo Villeneuve" <hugo@...ovil.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Jiri Slaby" <jirislaby@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-serial@...r.kernel.org>,
        "Linus Walleij" <linus.walleij@...aro.org>,
        "Gregory CLEMENT" <gregory.clement@...tlin.com>,
        "Alexandre Belloni" <alexandre.belloni@...tlin.com>,
        "Thomas Petazzoni" <thomas.petazzoni@...tlin.com>,
        "Vladimir Kondratiev" <vladimir.kondratiev@...ileye.com>,
        "Tawfik Bayouk" <tawfik.bayouk@...ileye.com>
Subject: Re: [PATCH 6/6] tty: serial: amba-pl011: Parse bits option as 5, 6,
 7 or 8 in _get_options

On Tue Oct 31, 2023 at 12:22 PM CET, Russell King (Oracle) wrote:
> On Tue, Oct 31, 2023 at 12:04:11PM +0100, Théo 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.
> > 
> > I see your point. Two things come to mind:
> > 
> >  - I added this parsing of 5/6 bits to be symmetrical with
> >    pl011_set_termios that handles 5/6 properly. Should pl011_set_termios
> >    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,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com