Re: /dev/ttynull and VT-less kernels on Desktop

[nerdopolis <bluescreen_avenger@verizon.net>]

On Monday, August 5, 2024 9:16:21 PM EDT nerdopolis wrote:
> On Sunday, August 4, 2024 5:45:07 PM EDT nerdopolis wrote:
> > Hi
> > 
> > I have been doing some testing on VT-less desktops. I brought up an issue with
> > how systemd was not logging to /dev/console because isatty() was failing (in a
> > thread last month which I kind of bungled by accidentally sending the first
> > message with rich text enabled by accident) I solved that issue though. As
> > disabling the VT console causes the default console to /dev/ttyS0 and the 
> > isatty() calls on it were returning false.
> > 
> > I have found out that this issue actually already has an existing solution in
> > the ttynull driver. After enabling it, and then adding console=ttynull to the
> > kernel command line, this allows isatty() to succeed on VT-less systems, and
> > now systemd is able to correctly log its status messages to the kernel console
> > on VT-less systems, without needing anything connected to /dev/ttyS0
> > 
> > However I have two questions:
> > 
> > 1. Is there a build option to make it prioritize /dev/ttynull over /dev/ttyS0 ?
> > 
Hi

So I was able to come up with a hackish tweak, making a CONFIG_NULL_TTY_CONSOLE
it would depend on CONFIG_NULL_TTY being enabled, and CONFIG_VT_CONSOLE being 
NOT enabled.

and then doing a 
-------------------------------------------------------------------------------
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index dddb15f48d59..6950118c6fb3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3728,6 +3728,10 @@ void __init console_init(void)
 		trace_initcall_finish(call, ret);
 		ce++;
 	}
+#ifdef CONFIG_NULL_TTY_CONSOLE
+	if (preferred_console < 0)
+		__add_preferred_console("ttynull", 0, NULL, NULL, false);
+#endif
 }
 
 /*
-------------------------------------------------------------------------------

It's probably not correct, but it works for me locally, after some grepping and
guessing...


Thanks
> >    It would probably be neeed so that distributions don't have to worry about
> >    the logistics of adding that to everyone's GRUB_CMDLINE_LINUX_DEFAULT come
> >    the eventual day the decide to go VT-less during its upgrade process.
> > 
> > 2. Is there a good way for the userspace to know that /dev/ttynull is primary?
> > 
> Hi
> 
> So I think I figured this part out, while ttynull doesn't make its way into the
> /sys/devices/virtual/tty/console/active file, it is reported in /proc/consoles
> so I will read from there, seems like the primary console is always the first
> line, so this part is solved
> 
> Thanks
> >    "ttynull" doesn't seem to appear in /sys/devices/virtual/tty/console/active
> >    when it is active. Which seems like it could make sense, (as in a check if
> >    it is empty) however if I boot with "console=ttyS0 console=ttynull", "ttyS0"
> >    appears in the file, but if I boot with say "console=tty0 console=ttyS0",
> >    they both appear in the file. It would be nice if in the first case it would
> >    report "ttyS0 ttynull".
> > 
> >    With Plymouth, when the primary console is /dev/ttyS0 instead of /dev/tty0
> >    it defaults to serial logging instead of showing a graphical splash, so now
> >    if you want a graphical splash on a VT-less system, you have to also specify
> >    "plymouth.graphical" on the kernel command line. If it could see "ttynull"
> >    in the end of the file, distributions that go VT-less can also just enable
> >    "ttynull" as the default console, and Plymouth could use that to
> >    differentiate between VT-less desktop systems, or systems that actually want
> >    serial logging, so they don't have to change GRUB_CMDLINE_LINUX_DEFAULT.
> > 
> > Thanks
> > 
> > 
> > 
> > 
> > 
> 
> 
> 
> 
>