linux-kernel - Re: [PATCH] serial: core: Add sysfs links for serial core port instances for ttys

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Date:   Fri, 21 Jul 2023 13:05:49 +0300
From:   Andy Shevchenko <andriy.shevchenko@...el.com>
To:     Tony Lindgren <tony@...mide.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>, Dhruva Gole <d-gole@...com>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        John Ogness <john.ogness@...utronix.de>,
        Johan Hovold <johan@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-omap@...r.kernel.org, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] serial: core: Add sysfs links for serial core port
 instances for ttys

On Thu, Jul 20, 2023 at 07:13:19AM +0300, Tony Lindgren wrote:
> * Andy Shevchenko <andriy.shevchenko@...el.com> [230719 13:59]:
> > On Wed, Jul 19, 2023 at 08:43:21AM +0300, Tony Lindgren wrote:
> > > * Andy Shevchenko <andriy.shevchenko@...el.com> [230719 05:34]:
> > > > On Wed, Jul 19, 2023 at 08:16:11AM +0300, Tony Lindgren wrote:

...

> > > > > And with this, we can add /dev/serial/by-id symlinks to the serial port
> > > > > device instances so we can start using serial core port addressing in
> > > > > addition to the legacy ttyS naming.
> > > > > 
> > > > > The naming we can use is dev_name:0.0 where 0.0 are the serial core
> > > > > controller id and port id, so for the ttyS0 example above the naming
> > > > > would be 00:04.0:0.0.
> > > > 
> > > > This is interesting idea. But any hint why it can be useful?
> > > 
> > > If you have lots of serial ports and we are stuck with adding aliases
> > > for the ports in the dts files where the ttyS naming and ordering does
> > > not really help or may not necessarily make sense if the ports are on
> > > different buses or domains. With CONFIG_SERIAL_8250_RUNTIME_UARTS=4,
> > > the ttyS naming is only needed for the legacy ports really.
> > 
> > I see. Does it fix the long standing issue with ttyS enumeration (on x86
> > at least) when depending on the presence of the legacy ports the HSUART
> > (high speed) can preempt the legacy placeholders (ttyS0..ttyS3)?
> >
> > To me sounds like it may very well do fix it and I would be glad to see that
> > in the commit message (as selling point) and in documentation.
> 
> It won't affect how ttyS0..ttyS3 get assigned, but it helps finding your
> HSUART instance with DEVNAME:0.0 style addressing. So you don't need to
> care what ttyS number the port has. If you have such a test case maybe give
> it a try.

Exactly, the problem (currently) is, that depending on the BIOS settings
the kernel can't use the same command line and this is quite a PITA for
our customers!

As far as we can specify hardware (path) to the console, it will be so
cool and hopefully solves this very long standing issue.

-- 
With Best Regards,
Andy Shevchenko