ACPI: SPCR: Consider baud rate 0 as preconfigured state

[ Upstream commit b413b1ab ]

Since SPCR 1.04 [1] the baud rate of 0 means a preconfigured state of UART.
Assume firmware or bootloader configures console correctly.

[1]: https://docs.microsoft.com/en-us/windows-hardware/drivers/serports/serial-port-console-redirection-tableSigned-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/acpi/spcr.c

@@ -148,6 +148,13 @@ int __init acpi_parse_spcr(bool enable_earlycon, bool enable_console)
 	}
 
 	switch (table->baud_rate) {
+	case 0:
+		/*
+		 * SPCR 1.04 defines 0 as a preconfigured state of UART.
+		 * Assume firmware or bootloader configures console correctly.
+		 */
+		baud_rate = 0;
+		break;
 	case 3:
 		baud_rate = 9600;
 		break;
@@ -196,6 +203,10 @@ int __init acpi_parse_spcr(bool enable_earlycon, bool enable_console)
 		 * UART so don't attempt to change to the baud rate state
 		 * in the table because driver cannot calculate the dividers
 		 */
+		baud_rate = 0;
+	}
+
+	if (!baud_rate) {
 		snprintf(opts, sizeof(opts), "%s,%s,0x%llx", uart, iotype,
 			 table->serial_port.address);
 	} else {