On Wed, 29 Mar 2017, Bruce Evans wrote:

On Wed, 29 Mar 2017, Andrey Chernov wrote:
Moreover, I can't enter KDB via Ctrl-Alt-ESC in the syscons only mode
anymore - nothing happens. In the vt mode I can, but can't exit via "c"
properly, all chars typed after "c" produce beep unless I switch to
another screen and back.

All it means that syscons becomes very broken now by itself and even
damages the kernel operations.

But I suspect it is a usb keyboard problem.  Syscons now does almost
correct locking for the screen, but not for the keyboard, and the usb
keyboard is especially fragile, especially in ddb mode.  Console input
is not used in normal operation except for checking for characters on

Try using vt with syscons unconfigured.  Syscons shouldn't be used when
vt is selected, but unconfigure it to be sure.  vt has different bugs
using the usb keyboard.  I haven't tested usb keyboards recently.

I tested usb keyboards again.  They sometimes work, much the same as
a few months ago after some fixes:
- after booting with -d, they never work (give no input) at the ddb
  prompt with either sc or vt.  usb is not initialized then, and no usb
  keyboard is attached to sc or vt
- after booting without loader with -a, sc rarely or never works (gives
  no input) at the mountroot prompt
- after booting with loader with -a, vt works at the mountroot prompt.
  I don't normally use loader but need to use it to change the configuration.
  This might be better than before.  There used to be a screen refresh bug.
- after booting with loader with -a, sc works at the mountroot prompt too.
  I previously debugged that vt worked better because it attaches the keyboard
  before this point, while sc attaches it after.  Booting with loader
  apparently fixes the order.
- after any booting, sc works for user input (except sometimes after a
  too-soft hard reset, the keyboard doesn't even work in the BIOS, and it
  takes unplugging the keyboard to fix this)
- after almost any booting, vt doesn't work for user input (gives no input).
  However, if ddb is entered using a serial console, vt does work!  A few
  months ago, normal input was fixed by configuring kbdmux (the default in
  GENERIC).  It is not fixed by unplugging the keyboard.  kbdmux has a known
  bug of not doing nested switching for the keyboard state.  Perhaps this
  "fixes" ddb mode.  But I would have expected it to break ddb mode.
- I didn't test sc after entering ddb, except early when it doesn't work.

The above testing is with a usb keyboard, no ps/2 keyboard, and no kbdmux.
Other combinations and dynamic switching move the bugs around, and a
serial console is needed to recover in cases where the bugs prevent any
keyboard input.

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