[PATCH v3] ACPI: SPCR: Support Precise Baud Rate filed

[cp0613@linux.alibaba.com]

From: Chen Pei <cp0613@linux.alibaba.com>

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>
---
 drivers/acpi/spcr.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/acpi/spcr.c b/drivers/acpi/spcr.c
index cd36a97b0ea2..fa12e740386d 100644
--- a/drivers/acpi/spcr.c
+++ b/drivers/acpi/spcr.c
@@ -146,7 +146,15 @@ 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.
-- 
2.49.0