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

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

[cp0613]

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

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

[Rafael J. Wysocki]

On Sat, Sep 13, 2025 at 9:08 AM <cp0613@linux.alibaba.com> wrote:
>
> 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.
> --

Applied as 6.18 material, thanks!