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

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Date:   Thu, 20 Jul 2023 07:13:19 +0300
From:   Tony Lindgren <tony@...mide.com>
To:     Andy Shevchenko <andriy.shevchenko@...el.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

* 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:
> > > > Let's allow the userspace to find out the tty name for a serial core
> > > > controller id if a tty exists. This can be done with:
> > > > 
> > > > $ grep DEVNAME /sys/bus/serial-base/devices/port*/tty/uevent
> > > > /sys/bus/serial-base/devices/port.00:04.0/tty/uevent:DEVNAME=ttyS0
> > > > /sys/bus/serial-base/devices/port.serial8250.1/tty/uevent:DEVNAME=ttyS1
> > > > /sys/bus/serial-base/devices/port.serial8250.2/tty/uevent:DEVNAME=ttyS2
> > > > /sys/bus/serial-base/devices/port.serial8250.3/tty/uevent:DEVNAME=ttyS3
> > > 
> > > What part is the controller ID here?
> > 
> > Oh looks like controller id it's missing in the name, I'll send a fix
> > for that.
> > 
> > > We also have something in procfs (I don't remember what info exactly is there).
> > 
> > Do you mean /proc/devices?
> 
> Something tty specific, /proc/tty/, but I had a look and it seems for another
> stuff.

OK

> > > > 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.

Regards,

Tony