[PATCH RESEND v2 0/2] ACPI: SPCR: Support UART clock frequency field

[PATCH RESEND v2 0/2] ACPI: SPCR: Support UART clock frequency field

[Markus Probst]

Support the uart clock frequency in the SPCR table.
See the commit messages for details.

Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
Changes in v2:
- fix uart_clk_freq possibly being interpreted as parity/bits/flow
- Link to v1: https://patch.msgid.link/20260505-acpi_spcr-v1-1-fd4bc6f4eb53@posteo.de

---
Markus Probst (2):
      serial: earlycon: add uart_clk_freq parameter
      ACPI: SPCR: Support UART clock frequency field

 arch/m68k/virt/config.c          |  2 +-
 arch/mips/mti-malta/malta-init.c |  2 +-
 drivers/acpi/spcr.c              |  2 +-
 drivers/firmware/efi/earlycon.c  |  2 +-
 drivers/tty/serial/earlycon.c    | 17 ++++++++++++-----
 include/linux/serial_core.h      |  7 +++++--
 6 files changed, 21 insertions(+), 11 deletions(-)
---
base-commit: aa61612ab641d7d62b0b6889f2c7c9251489f6e3
change-id: 20260430-acpi_spcr-61902fd923f2
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-- 
Markus Probst <markus.probst@posteo.de>

[PATCH RESEND v2 1/2] serial: earlycon: add uart_clk_freq parameter

[Markus Probst]

Add `uart_clk_freq` parameter to `setup_earlycon`. This allows the
options string to be reused with `add_preferred_console`, while still
allowing to set the uart clock frequency. This will be used in the
following commit ("ACPI: SPCR: Support UART clock frequency field").

No logical change intended.

Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
 arch/m68k/virt/config.c          |  2 +-
 arch/mips/mti-malta/malta-init.c |  2 +-
 drivers/acpi/spcr.c              |  2 +-
 drivers/firmware/efi/earlycon.c  |  2 +-
 drivers/tty/serial/earlycon.c    | 17 ++++++++++++-----
 include/linux/serial_core.h      |  7 +++++--
 6 files changed, 21 insertions(+), 11 deletions(-)

diff --git a/arch/m68k/virt/config.c b/arch/m68k/virt/config.c
index b338e2a8da6a..2c35ec15a51b 100644
--- a/arch/m68k/virt/config.c
+++ b/arch/m68k/virt/config.c
@@ -83,7 +83,7 @@ void __init config_virt(void)
 
 	snprintf(earlycon, sizeof(earlycon), "early_gf_tty,0x%08x",
 		 virt_bi_data.tty.mmio);
-	setup_earlycon(earlycon);
+	setup_earlycon(earlycon, 0);
 
 	mach_init_IRQ = virt_init_IRQ;
 	mach_sched_init = virt_sched_init;
diff --git a/arch/mips/mti-malta/malta-init.c b/arch/mips/mti-malta/malta-init.c
index 82b0fd8576a2..88ef17967ced 100644
--- a/arch/mips/mti-malta/malta-init.c
+++ b/arch/mips/mti-malta/malta-init.c
@@ -75,7 +75,7 @@ static void __init console_config(void)
 	if ((strstr(fw_getcmdline(), "earlycon=")) == NULL) {
 		sprintf(console_string, "uart8250,io,0x3f8,%d%c%c", baud,
 			parity, bits);
-		setup_earlycon(console_string);
+		setup_earlycon(console_string, 0);
 	}
 
 	if ((strstr(fw_getcmdline(), "console=")) == NULL) {
diff --git a/drivers/acpi/spcr.c b/drivers/acpi/spcr.c
index 73cb933fdc89..cfacbe53f279 100644
--- a/drivers/acpi/spcr.c
+++ b/drivers/acpi/spcr.c
@@ -228,7 +228,7 @@ int __init acpi_parse_spcr(bool enable_earlycon, bool enable_console)
 	pr_info("console: %s\n", opts);
 
 	if (enable_earlycon)
-		setup_earlycon(opts);
+		setup_earlycon(opts, 0);
 
 	if (enable_console)
 		err = add_preferred_console(uart, 0, opts + strlen(uart) + 1);
diff --git a/drivers/firmware/efi/earlycon.c b/drivers/firmware/efi/earlycon.c
index 3d060d59968c..0e3c2cb08966 100644
--- a/drivers/firmware/efi/earlycon.c
+++ b/drivers/firmware/efi/earlycon.c
@@ -221,7 +221,7 @@ static bool __initdata fb_probed;
 void __init efi_earlycon_reprobe(void)
 {
 	if (fb_probed)
-		setup_earlycon("efifb");
+		setup_earlycon("efifb", 0);
 }
 
 static int __init efi_earlycon_setup(struct earlycon_device *device,
diff --git a/drivers/tty/serial/earlycon.c b/drivers/tty/serial/earlycon.c
index ab9af37f6cda..a419943e083b 100644
--- a/drivers/tty/serial/earlycon.c
+++ b/drivers/tty/serial/earlycon.c
@@ -135,11 +135,14 @@ static int __init parse_options(struct earlycon_device *device, char *options)
 	return 0;
 }
 
-static int __init register_earlycon(char *buf, const struct earlycon_id *match)
+static int __init register_earlycon(char *buf, unsigned int uart_clk_freq,
+				    const struct earlycon_id *match)
 {
 	int err;
 	struct uart_port *port = &early_console_dev.port;
 
+	port->uartclk = uart_clk_freq;
+
 	/* On parsing error, pass the options buf to the setup function */
 	if (buf && !parse_options(&early_console_dev, buf))
 		buf = NULL;
@@ -164,7 +167,8 @@ static int __init register_earlycon(char *buf, const struct earlycon_id *match)
 
 /**
  *	setup_earlycon - match and register earlycon console
- *	@buf:	earlycon param string
+ *	@buf:		earlycon param string
+ *	@uart_clk_freq:	uart clock frequency in Hz or 0 for BASE_BAUD*16
  *
  *	Registers the earlycon console matching the earlycon specified
  *	in the param string @buf. Acceptable param strings are of the form
@@ -177,10 +181,13 @@ static int __init register_earlycon(char *buf, const struct earlycon_id *match)
  *	<options> string in the 'options' parameter; all other forms set
  *	the parameter to NULL.
  *
+ *	If the uart clock frequency is specified in the 'options' parameter,
+ *	the value of the param @uart_clk_freq will be ignored.
+ *
  *	Returns 0 if an attempt to register the earlycon was made,
  *	otherwise negative error code
  */
-int __init setup_earlycon(char *buf)
+int __init setup_earlycon(char *buf, unsigned int uart_clk_freq)
 {
 	const struct earlycon_id *match;
 	bool empty_compatible = true;
@@ -209,7 +216,7 @@ int __init setup_earlycon(char *buf)
 		} else
 			buf = NULL;
 
-		return register_earlycon(buf, match);
+		return register_earlycon(buf, uart_clk_freq, match);
 	}
 
 	if (empty_compatible) {
@@ -241,7 +248,7 @@ static int __init param_setup_earlycon(char *buf)
 		}
 	}
 
-	err = setup_earlycon(buf);
+	err = setup_earlycon(buf, 0);
 	if (err == -ENOENT || err == -EALREADY)
 		return 0;
 	return err;
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 666430b47899..5c60fda9dd3a 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -1097,10 +1097,13 @@ int of_setup_earlycon(const struct earlycon_id *match, unsigned long node,
 
 #ifdef CONFIG_SERIAL_EARLYCON
 extern bool earlycon_acpi_spcr_enable __initdata;
-int setup_earlycon(char *buf);
+int setup_earlycon(char *buf, unsigned int uart_clk_freq);
 #else
 static const bool earlycon_acpi_spcr_enable EARLYCON_USED_OR_UNUSED;
-static inline int setup_earlycon(char *buf) { return 0; }
+static inline int setup_earlycon(char *buf, unsigned int uart_clk_freq)
+{
+	return 0;
+}
 #endif
 
 /* Variant of uart_console_registered() when the console_list_lock is held. */

-- 
2.53.0

[PATCH RESEND v2 2/2] ACPI: SPCR: Support UART clock frequency field

[Markus Probst]

The Microsoft Serial Port Console Redirection (SPCR) specification
revision 1.08 comprises additional field: UART Clock Frequency [1].

It contains a non-zero value indicating the UART clock frequency in Hz.

Link: https://learn.microsoft.com/en-us/windows-hardware/drivers/serports/serial-port-console-redirection-table [1]
Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
 drivers/acpi/spcr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/acpi/spcr.c b/drivers/acpi/spcr.c
index cfacbe53f279..16f94073fde6 100644
--- a/drivers/acpi/spcr.c
+++ b/drivers/acpi/spcr.c
@@ -228,7 +228,7 @@ int __init acpi_parse_spcr(bool enable_earlycon, bool enable_console)
 	pr_info("console: %s\n", opts);
 
 	if (enable_earlycon)
-		setup_earlycon(opts, 0);
+		setup_earlycon(opts, table->header.revision >= 3 ? table->uart_clk_freq : 0);
 
 	if (enable_console)
 		err = add_preferred_console(uart, 0, opts + strlen(uart) + 1);

-- 
2.53.0

Re: [PATCH RESEND v2 1/2] serial: earlycon: add uart_clk_freq parameter

[Greg Kroah-Hartman]

On Mon, Jun 08, 2026 at 10:40:21PM +0000, Markus Probst wrote:
> Add `uart_clk_freq` parameter to `setup_earlycon`. This allows the
> options string to be reused with `add_preferred_console`, while still
> allowing to set the uart clock frequency. This will be used in the
> following commit ("ACPI: SPCR: Support UART clock frequency field").

Ick, this is bad, now you need to look up what this 0 is as a parameter
every time you see this call.  Please just add a new function that takes
the new paramter, don't abuse the old one for this.

thanks,

greg k-h

Re: [PATCH RESEND v2 1/2] serial: earlycon: add uart_clk_freq parameter

[Geert Uytterhoeven]

Hi Markus,

On Tue, 9 Jun 2026 at 00:40, Markus Probst <markus.probst@posteo.de> wrote:
> Add `uart_clk_freq` parameter to `setup_earlycon`. This allows the
> options string to be reused with `add_preferred_console`, while still
> allowing to set the uart clock frequency. This will be used in the
> following commit ("ACPI: SPCR: Support UART clock frequency field").
>
> No logical change intended.
>
> Signed-off-by: Markus Probst <markus.probst@posteo.de>

> --- a/drivers/tty/serial/earlycon.c
> +++ b/drivers/tty/serial/earlycon.c
> @@ -135,11 +135,14 @@ static int __init parse_options(struct earlycon_device *device, char *options)
>         return 0;
>  }
>
> -static int __init register_earlycon(char *buf, const struct earlycon_id *match)
> +static int __init register_earlycon(char *buf, unsigned int uart_clk_freq,
> +                                   const struct earlycon_id *match)
>  {
>         int err;
>         struct uart_port *port = &early_console_dev.port;
>
> +       port->uartclk = uart_clk_freq;

Who is actually consuming this value?
Earlycon typically works with the serial console, as configured before
Linux boot by the firmware.
The Microsoft doc referenced in patch 2 seem to agree with that:

   "On a system where the BIOS or system firmware uses the serial
    port for console input/output, this table should be used to convey
    information about the settings, to ensure a seamless transition
    between the firmware console output and Windows EMS output."

> +
>         /* On parsing error, pass the options buf to the setup function */
>         if (buf && !parse_options(&early_console_dev, buf))
>                 buf = NULL;

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds