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/dev/ttynull and VT-less kernels on Desktop

DISCUSSION nerdopolis 1 participant 3 messages Aug 4, 2024 – Aug 10, 2024 1 message not in the archive

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nerdopolis

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#1 lore raw

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 ?

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?

"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

author
#2 lore raw
Quoted 17 lines · EDT nerdopolis

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:

Quoted 8 lines

1. Is there a build option to make it prioritize /dev/ttynull over /dev/ttyS0 ?

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

Quoted 22 lines

"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

author
#3 lore raw
Quoted 16 lines · EDT nerdopolis
Quoted 15 lines · EDT nerdopolis

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

Quoted 4 lines
Quoted 4 lines

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
-------------------------------------------------------------------------------

kernel/printk/printk.c +4 -0
index dddb15f48d59..6950118c6fb3 100644
@@ -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

Quoted 41 lines
Quoted 6 lines

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

Quoted 22 lines

"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