serial: Z85C30: avoid a hang at console switch-over

Changes to the generic console support code that happened a while ago
introduced a scenario where the initial console is used in parallel with
the final console during a brief period when switching between the two is
in progress. During that time a message about the switch-over is printed.

With some combinations of chips, firmware and drivers, such as the Zilog
Z85C30 SCC used with the DECstation, a hang may happen because the
firmware used for the initial console may not expect the state of the chip
after it has been initialised by the driver. This is not a bug in the
firmware, as some registers it would have to examine are write-only.

This is a workaround for the Z85C30 which reuses the power-management
callback to keep the transmitter of the line associated with the console
enabled. It reflects the consensus reached in a discussion a while ago.

Signed-off-by: Maciej W. Rozycki <[e-mail]>
Cc: Jiri Slaby <[e-mail]>
Cc: Alan Cox <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/serial/zs.c

···
 	zport->regs[1] &= ~RxINT_MASK;
 	zport->regs[1] |= RxINT_ALL | TxINT_ENAB | EXT_INT_ENAB;
 	zport->regs[3] |= RxENABLE;
-	zport->regs[5] |= TxENAB;
 	zport->regs[15] |= BRKIE;
 	write_zsreg(zport, R1, zport->regs[1]);
 	write_zsreg(zport, R3, zport->regs[3]);
···
 
 	spin_lock_irqsave(&scc->zlock, flags);
 
-	zport->regs[5] &= ~TxENAB;
 	zport->regs[3] &= ~RxENABLE;
 	write_zsreg(zport, R5, zport->regs[5]);
 	write_zsreg(zport, R3, zport->regs[3]);
···
 	spin_unlock_irqrestore(&scc->zlock, flags);
 }
 
+/*
+ * Hack alert!
+ * Required solely so that the initial PROM-based console
+ * works undisturbed in parallel with this one.
+ */
+static void zs_pm(struct uart_port *uport, unsigned int state,
+		  unsigned int oldstate)
+{
+	struct zs_port *zport = to_zport(uport);
+
+	if (state < 3)
+		zport->regs[5] |= TxENAB;
+	else
+		zport->regs[5] &= ~TxENAB;
+	write_zsreg(zport, R5, zport->regs[5]);
+}
+
 
 static const char *zs_type(struct uart_port *uport)
 {
···
 	.startup	= zs_startup,
 	.shutdown	= zs_shutdown,
 	.set_termios	= zs_set_termios,
+	.pm		= zs_pm,
 	.type		= zs_type,
 	.release_port	= zs_release_port,
 	.request_port	= zs_request_port,
···
 		return ret;
 
 	zs_reset(zport);
+	zs_pm(uport, 0, -1);
 
 	if (options)
 		uart_parse_options(options, &baud, &parity, &bits, &flow);