braille: nbcon: Allow to use a serial console with NBCON API as Braille console

The Braille console is not registered in console_list. Instead, it is
integrated with the virtual terminal (VT) and shows what is displayed
on the terminal. It writes the data using con->write*() callback
of the associated serial console driver.

braille_write() is called from the VT code under console_lock().
The associated serial console driver can be converted to the NBCON API
though. The situation is similar to flushing nbcon consoles in the legacy
loop when some boot consoles are still registered, see
nbcon_legacy_emit_next_record().

But there is a big difference though. braille_write() is not directly
called from the code paths flushing registered consoles. The VT code
expects that braille_write() succeeds. It does not replay the message
when it can't acquire the ownership. As a result, braille_write():

  + must try harder to get the ownership.

  + has to be synchronized only against non-printk serial console
    which depend nbcon_device_try_acquire() using NBCON_PRIO_NORMAL.

Let's look at it from another side and try to simulate the original
locking using the NBCON API:

  1. Take con->device_lock(), aka the port->lock in the legacy serial
     console driver.

  2. Acquire nbcon context to provide some synchronization for a panic()
     context. Use NBCON_PRIO_NORMAL because it contends only with
     nbcon_device_try_acquire() users. Do it in a busy loop. It should
     always succeed when con->device_lock() succeeded because all other
     users do the same. The only exception is when the context get acquired
     by a CPU handling panic.

  3. In panic, disable interrupts and try to acquire the nbcon context.
     Use NBCON_PRIO_PANIC. And try even an unsafe takeover because
     otherwise the Braille console won't see the text shown during panic().
     It is similar to the oops_in_progress/trylock handling in the legacy
     serial console driver.

Finally, avoid the newline prepending logic in the existing serial console
drivers when they are used as a Braille console. As explained above,
the Braille console shows the last modified line on the terminal (VT).
braille_write() is called when single characters are added. Most
messages are not ended by newline. Anyway, the VT code does not have
logic to reply partially printed messages.

Fixes: 13189fa73afa ("printk: nbcon: Rely on kthreads for normal operation")
Reviewed-by: John Ogness <john.ogness@linutronix.de>
[pmladek@suse.com: Remove white space changes in braille_register_console(). Typo fix.]
Link: https://patch.msgid.link/20261001140727.124398-2-pmladek@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>

0 files changed, 0 insertions, 0 deletions