serial: 8250: Switch to nbcon console

Implement the necessary callbacks to switch the 8250 console driver
to perform as an nbcon console.

Add implementations for the nbcon console callbacks:

  ->write_atomic()
  ->write_thread()
  ->device_lock()
  ->device_unlock()

and add CON_NBCON to the initial @flags.

All register access in the callbacks are within unsafe sections.
The ->write_atomic() and ->write_thread() callbacks allow safe
handover/takeover per byte and add a preceding newline if they
take over from another context mid-line.

For the ->write_atomic() callback, a new irq_work is used to defer
modem control since it may be called from a context that does not
allow waking up tasks.

Note: A new __serial8250_clear_IER() is introduced for direct
clearing of UART_IER. This will allow to restore the lockdep
check to serial8250_clear_IER() in a follow-up commit.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20250107212702.169493-6-john.ogness@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 11 insertions, 2 deletions

diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index 144de7a7948d..57875c37023a 100644
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -150,8 +150,17 @@ struct uart_8250_port {
 #define LSR_SAVE_FLAGS UART_LSR_BRK_ERROR_BITS
 	u16			lsr_saved_flags;
 	u16			lsr_save_mask;
+
+	/*
+	 * Track when a console line has been fully written to the
+	 * hardware, i.e. true when the most recent byte written to
+	 * UART_TX by the console was '\n'.
+	 */
+	bool			console_line_ended;
+
 #define MSR_SAVE_FLAGS UART_MSR_ANY_DELTA
 	unsigned char		msr_saved_flags;
+	struct irq_work		modem_status_work;
 
 	struct uart_8250_dma	*dma;
 	const struct uart_8250_ops *ops;
@@ -202,8 +211,8 @@ void serial8250_tx_chars(struct uart_8250_port *up);
 unsigned int serial8250_modem_status(struct uart_8250_port *up);
 void serial8250_init_port(struct uart_8250_port *up);
 void serial8250_set_defaults(struct uart_8250_port *up);
-void serial8250_console_write(struct uart_8250_port *up, const char *s,
-			      unsigned int count);
+void serial8250_console_write(struct uart_8250_port *up,
+			      struct nbcon_write_context *wctxt, bool in_atomic);
 int serial8250_console_setup(struct uart_port *port, char *options, bool probe);
 int serial8250_console_exit(struct uart_port *port);