linux-kernel - Re: [PATCH v2] ACPI: SPCR: Support Precise Baud Rate filed

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Message-ID: <CAJF2gTTn6PhqjOPAxVfSy_8avb1_7BAeqOsFxPnvmZ3KF8sqog@mail.gmail.com>
Date: Sat, 13 Sep 2025 13:24:21 +0800
From: Guo Ren <guoren@...nel.org>
To: cp0613@...ux.alibaba.com
Cc: rafael@...nel.org, lenb@...nel.org, jeeheng.sia@...rfivetech.com, 
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ACPI: SPCR: Support Precise Baud Rate filed

On Fri, Sep 12, 2025 at 8:25 PM <cp0613@...ux.alibaba.com> wrote:
>
> From: Chen Pei <cp0613@...ux.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
The spec states that it would "override" the baud rate. That means it
should happen behind the table->baud_rate or firmware still needs to
give out a legal Configured Baud Rate for precise_baudrate.

Then, we move these:
+       if (table->precise_baudrate)
+               baud_rate = table->precise_baudrate;
behind the "switch (table->baud_rate) {}".

Right?

> 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@...ux.alibaba.com>
> ---
>  drivers/acpi/spcr.c | 54 ++++++++++++++++++++++++++-------------------
>  1 file changed, 31 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/acpi/spcr.c b/drivers/acpi/spcr.c
> index cd36a97b0ea2..a97b02ee5538 100644
> --- a/drivers/acpi/spcr.c
> +++ b/drivers/acpi/spcr.c
> @@ -146,29 +146,37 @@ int __init acpi_parse_spcr(bool enable_earlycon, bool enable_console)
>                 goto done;
>         }
>
> -       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;
> -       case 4:
> -               baud_rate = 19200;
> -               break;
> -       case 6:
> -               baud_rate = 57600;
> -               break;
> -       case 7:
> -               baud_rate = 115200;
> -               break;
> -       default:
> -               err = -ENOENT;
> -               goto done;
> +       /*
> +        * 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.
> +                        * Assume firmware or bootloader configures console correctly.
> +                        */
> +                       baud_rate = 0;
> +                       break;
> +               case 3:
> +                       baud_rate = 9600;
> +                       break;
> +               case 4:
> +                       baud_rate = 19200;
> +                       break;
> +               case 6:
> +                       baud_rate = 57600;
> +                       break;
> +               case 7:
> +                       baud_rate = 115200;
> +                       break;
> +               default:
> +                       err = -ENOENT;
> +                       goto done;
>         }
>
>         /*
> --
> 2.49.0
>


-- 
Best Regards
 Guo Ren