Messages in this thread | | | From | Guillermo Rodriguez Garcia <> | Date | Wed, 22 Mar 2023 09:33:32 +0100 | Subject | Re: Change of behaviour when console=null and ttynull driver is used |
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Hi Petr,
El vie, 17 mar 2023 a las 13:51, Petr Mladek (<pmladek@suse.com>) escribió: > > On Thu 2023-03-16 11:29:26, Guillermo Rodriguez Garcia wrote: > > Hi all, > > > > We have several embedded systems where pass console= or console=null > > in production to disable the console. > > > > Later we check for this in user space: in our inittab we check if fd0 > > is "associated with a terminal" (test -t 0); if so, we are in > > development mode and we open a debug shell; otherwise (console > > disabled) we just start the application. > > > > Recently [1] this behaviour has changed and now if we pass console= or > > console=null, the new ttynull driver is used. This breaks the check we > > were doing (test -t 0 always true now). > > > > [1]: https://lore.kernel.org/lkml/X%2FcDG%2FxCCzSWW2cd@alley/t/ > > This is actually exactly the problem that the change tried to solve. > Some systems failed to boot when there was no console and they tried > to write something at stdout.
Well, I did not have any problem before this change.
I understand that some systems had a problem (many others didn't, and setting console= or console=null has been standard practice for a long time). Since this change in behaviour could (and did) break things in user space, perhaps it should have been made opt-in...
> > > Is there a way to get the previous behaviour? If not, is there an easy > > way for userspace to detect whether the console device is a "real" tty > > or ttynull (other than trying to parse the kernel boot args, which > > would be a bit fragile). > > A solution would be to check that /proc/consoles has ttynull as the > only registred console, for example: > > grep -q ttynull /proc/consoles && test `cat /proc/consoles | wc -l` -eq 1 > > Would this work for you, please?
I was trying to avoid something like this as it feels like userspace now needs to have too much knowledge of what the kernel is doing internally, however I guess this is the best option.
The suggested check seems to work but now I am seeing a different (related?) issue: when I try to reboot from the console (using busybox's reboot command), the system reboots normally if I am using a "normal" console (e.g. console=ttyXXX), however when using console=ttynull, the command takes 20-25 seconds to complete. I am not sure why this would happen; if I understand correctly, for userspace ttynull is just like a regular tty driver, so why would there be a difference?
Any ideas?
Thanks,
Guillermo Rodriguez Garcia guille.rodriguez@gmail.com
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