ACPI: SPCR: Support Precise Baud Rate field

The Microsoft Serial Port Console Redirection (SPCR) specification
revision 1.09 comprises additional field: Precise Baud Rate [1].

It is used to describe non-traditional baud rates (such as those
used by high-speed UARTs).

It contains a specific non-zero baud rate which overrides the value
of the Configured Baud Rate field. If this field is zero or not
present, Configured Baud Rate is used.

Link: https://learn.microsoft.com/en-us/windows-hardware/drivers/serports/serial-port-console-redirection-table [1]
Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
Link: https://patch.msgid.link/20250913070815.16758-1-cp0613@linux.alibaba.com
[ rjw: Corrected typo in the subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: cp0613

drivers/acpi/spcr.c

@@ -149,7 +149,15 @@ int __init acpi_parse_spcr(bool enable_earlycon, bool enable_console)
 		goto done;
 	}
 
-	switch (table->baud_rate) {
+	/*
+	 * SPCR 1.09 defines Precise Baud Rate Filed contains a specific
+	 * non-zero baud rate which overrides the value of the Configured
+	 * Baud Rate field. If this field is zero or not present, Configured
+	 * Baud Rate is used.
+	 */
+	if (table->precise_baudrate)
+		baud_rate = table->precise_baudrate;
+	else switch (table->baud_rate) {
 	case 0:
 		/*
 		 * SPCR 1.04 defines 0 as a preconfigured state of UART.