[RFC PATCH v2 0/3] Prefer working VT console over SPCR and device-tree chosen stdout-path

[Alper Nebi Yasak <alpernebiyasak@gmail.com>]

I recently experienced some trouble with setting up an encrypted-root
system, my Chromebook Plus (rk3399-gru-kevin, ARM64) would appear to
hang where it should have asked for an encryption passphrase; and I
eventually figured out that the kernel preferred the serial port
(inaccessible to me) over the built-in working display/keyboard and was
probably asking there.

Running plymouth in the initramfs solves that specific problem, but
both the documentation and tty-related kconfig descriptions imply that
/dev/console should be tty0 if graphics are working, CONFIG_VT_CONSOLE
is enabled and no explicit console argument is given in the kernel
commandline.

However, I'm seeing different behaviour on systems with SPCR (as in QEMU
aarch64 virtual machines) and/or a device-tree chosen stdout-path node
(as in most arm/arm64 devices). On these machines, depending on the
console argument, the contents of the /proc/consoles file are:

                    |     "console=tty0"    |    (no console arg)   |
  ------------------+-----------------------+-----------------------+
  QEMU VM           | tty0     -WU (EC p  ) | ttyAMA0  -W- (EC   a) |
  (w/ SPCR)         | ttyAMA0  -W- (E    a) |                       |
  ------------------+-----------------------+-----------------------+
  Chromebook Plus   | tty0     -WU (EC p  ) | ttyS2    -W- (EC p a) |
  (w/ stdout-path)  |                       | tty0     -WU (E     ) |
  ------------------+-----------------------+-----------------------+
  Chromebook Plus   | tty0     -WU (EC p  ) | tty0     -WU (EC p  ) |
  (w/o either)      |                       |                       |
  ------------------+-----------------------+-----------------------+

This patchset tries to ensure that VT is preferred in those conditions
even in the presence of firmware-mandated serial consoles. These should
cleanly apply onto next-20200430.

More discussion due to or about the console confusion on ARM64:
- My Debian bug report about the initramfs prompts [0]
- Fedora test issue arising from ARM64 QEMU machines having SPCR [1]
- Debian-installer discussion on what to do with multiple consoles [2]

[0] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=952452
[1] https://bugzilla.redhat.com/show_bug.cgi?id=1661288
[2] https://lists.debian.org/debian-boot/2019/01/msg00184.html

Changes in v2:
- Fix #elif to #else (Reported-by: kbuild test robot <lkp@intel.com>)
- Refresh dmesg outputs with/without earlycon for next-20200430
- Use the correct format when referencing a commit

Alper Nebi Yasak (3):
  printk: Add function to set console to preferred console's driver
  vt: Set as preferred console when a non-dummy backend is bound
  printk: Preset tty0 as a pseudo-preferred console

 drivers/tty/vt/vt.c     |  7 +++++
 include/linux/console.h |  1 +
 kernel/printk/printk.c  | 68 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 76 insertions(+)

-- 
2.26.2