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

[Théo Lebrun]

Hello,

On Tue Oct 31, 2023 at 3:05 PM CET, Russell King (Oracle) wrote:
> On Tue, Oct 31, 2023 at 02:51:45PM +0100, Théo Lebrun wrote:
> > 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.
>
> Precisely.
>
> > 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.
>
> I think you're conflating "serial console" with "serial port". A
> "serial port" can support multiple different formats, and in this case,
> such as 5, 6, 7, and 8 bits. 5 and 6 bits are likely to be a specialised
> application which uses a binary protocol, not ASCII.
>
> A "serial console" is one application of a "serial port" and a "serial
> console" is used to send ASCII characters, not a binary protocol.

That was all clear in my mind; I was missing the following bit:

> Sorry, but no, we don't assume every serial port is a serial console.
> Unless something has changed since I was involved with the serial
> layer, **we only read the parameters from a serial port _if_ and only
> if that port is being used as a serial console.**

Thank you for the time you took; I'll get rid of the patch and send a V2
fixing nits for other patches.

Regards,

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