[PATCH v3 0/2] ACPI: SPCR: Support UART clock frequency field

[PATCH v3 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 v3:
- add separate function for earlycon with uartclk
- Link to v2: https://patch.msgid.link/20260525-acpi_spcr-v2-0-c042089d73ca@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

 drivers/acpi/spcr.c           |  3 ++-
 drivers/tty/serial/earlycon.c | 17 ++++++++++++-----
 include/linux/serial_core.h   | 11 +++++++++--
 3 files changed, 23 insertions(+), 8 deletions(-)
---
base-commit: b3f94b2b3f3e51ab880a51fc6510e1dafba654ed
change-id: 20260430-acpi_spcr-61902fd923f2

[PATCH v3 1/2] serial: earlycon: add uart_clk_freq parameter

[Markus Probst]

Add function `setup_earlycon_with_uartclk`. 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>
---
 drivers/tty/serial/earlycon.c | 17 ++++++++++++-----
 include/linux/serial_core.h   | 11 +++++++++--
 2 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/drivers/tty/serial/earlycon.c b/drivers/tty/serial/earlycon.c
index ab9af37f6cda..5a20fe9e3fb6 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;
@@ -163,8 +166,9 @@ static int __init register_earlycon(char *buf, const struct earlycon_id *match)
 }
 
 /**
- *	setup_earlycon - match and register earlycon console
- *	@buf:	earlycon param string
+ *	setup_earlycon_with_uartclk - match and register earlycon console
+ *	@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_with_uartclk(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) {
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 110ad4e2aef9..19d8181c9005 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -1097,11 +1097,18 @@ 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_with_uartclk(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_with_uartclk(char *buf, unsigned int uart_clk_freq)
+{
+	return 0;
+}
 #endif
+static inline int setup_earlycon(char *buf)
+{
+	return setup_earlycon_with_uartclk(buf, 0);
+}
 
 /* Variant of uart_console_registered() when the console_list_lock is held. */
 static inline bool uart_console_registered_locked(struct uart_port *port)

-- 
2.53.0

[PATCH v3 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 | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/acpi/spcr.c b/drivers/acpi/spcr.c
index 73cb933fdc89..c79c809f49d4 100644
--- a/drivers/acpi/spcr.c
+++ b/drivers/acpi/spcr.c
@@ -228,7 +228,8 @@ 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_with_uartclk(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 v3 2/2] ACPI: SPCR: Support UART clock frequency field

[Greg Kroah-Hartman]

On Mon, Jun 15, 2026 at 12:40:22AM +0000, Markus Probst wrote:
> 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.

Ok, but then what are we going to do with this?  Please make this a lot
more descriptive, I have no idea what this is supposed to be doing at
all.

> 
> 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 | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/acpi/spcr.c b/drivers/acpi/spcr.c
> index 73cb933fdc89..c79c809f49d4 100644
> --- a/drivers/acpi/spcr.c
> +++ b/drivers/acpi/spcr.c
> @@ -228,7 +228,8 @@ 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_with_uartclk(opts,
> +					    table->header.revision >= 3 ? table->uart_clk_freq : 0);

Why the magic number 3?

thanks,
greg k-h

Re: [PATCH v3 1/2] serial: earlycon: add uart_clk_freq parameter

[Greg Kroah-Hartman]

On Mon, Jun 15, 2026 at 12:40:21AM +0000, Markus Probst wrote:
> Add function `setup_earlycon_with_uartclk`. 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>
> ---
>  drivers/tty/serial/earlycon.c | 17 ++++++++++++-----
>  include/linux/serial_core.h   | 11 +++++++++--
>  2 files changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/tty/serial/earlycon.c b/drivers/tty/serial/earlycon.c
> index ab9af37f6cda..5a20fe9e3fb6 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;

This looks like a logical change, the code today does not do that
anywhere, right?

thanks,

greg k-h

Re: [PATCH v3 1/2] serial: earlycon: add uart_clk_freq parameter

[Markus Probst]

[-- Attachment #1: Type: text/plain, Size: 1836 bytes --]

On Fri, 2026-07-10 at 14:36 +0200, Greg Kroah-Hartman wrote:
> On Mon, Jun 15, 2026 at 12:40:21AM +0000, Markus Probst wrote:
> > Add function `setup_earlycon_with_uartclk`. 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>
> > ---
> >  drivers/tty/serial/earlycon.c | 17 ++++++++++++-----
> >  include/linux/serial_core.h   | 11 +++++++++--
> >  2 files changed, 21 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/tty/serial/earlycon.c b/drivers/tty/serial/earlycon.c
> > index ab9af37f6cda..5a20fe9e3fb6 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;
> 
> This looks like a logical change, the code today does not do that
> anywhere, right?

`register_earlycon` only gets executed once.
Thus early_console_dev.port->uartclk is always 0.
Since all current callers in this patch series use `setup_earlycon`,
the uart_clk_freq parameter is also always 0.

So in case of behaviour/logic, this line does nothing without the 2.
patch.

Thanks
- Markus Probst

> 
> thanks,
> 
> greg k-h

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Re: [PATCH v3 1/2] serial: earlycon: add uart_clk_freq parameter

[Greg Kroah-Hartman]

On Sun, Jul 12, 2026 at 04:36:17PM +0000, Markus Probst wrote:
> On Fri, 2026-07-10 at 14:36 +0200, Greg Kroah-Hartman wrote:
> > On Mon, Jun 15, 2026 at 12:40:21AM +0000, Markus Probst wrote:
> > > Add function `setup_earlycon_with_uartclk`. 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>
> > > ---
> > >  drivers/tty/serial/earlycon.c | 17 ++++++++++++-----
> > >  include/linux/serial_core.h   | 11 +++++++++--
> > >  2 files changed, 21 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/drivers/tty/serial/earlycon.c b/drivers/tty/serial/earlycon.c
> > > index ab9af37f6cda..5a20fe9e3fb6 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;
> > 
> > This looks like a logical change, the code today does not do that
> > anywhere, right?
> 
> `register_earlycon` only gets executed once.
> Thus early_console_dev.port->uartclk is always 0.
> Since all current callers in this patch series use `setup_earlycon`,
> the uart_clk_freq parameter is also always 0.
> 
> So in case of behaviour/logic, this line does nothing without the 2.
> patch.

Then it belongs in the second patch, right?

thanks,

greg k-h

Re: [PATCH v3 1/2] serial: earlycon: add uart_clk_freq parameter

[Markus Probst]

[-- Attachment #1: Type: text/plain, Size: 2430 bytes --]

On Sun, 2026-07-12 at 18:40 +0200, Greg Kroah-Hartman wrote:
> On Sun, Jul 12, 2026 at 04:36:17PM +0000, Markus Probst wrote:
> > On Fri, 2026-07-10 at 14:36 +0200, Greg Kroah-Hartman wrote:
> > > On Mon, Jun 15, 2026 at 12:40:21AM +0000, Markus Probst wrote:
> > > > Add function `setup_earlycon_with_uartclk`. 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>
> > > > ---
> > > >  drivers/tty/serial/earlycon.c | 17 ++++++++++++-----
> > > >  include/linux/serial_core.h   | 11 +++++++++--
> > > >  2 files changed, 21 insertions(+), 7 deletions(-)
> > > > 
> > > > diff --git a/drivers/tty/serial/earlycon.c b/drivers/tty/serial/earlycon.c
> > > > index ab9af37f6cda..5a20fe9e3fb6 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;
> > > 
> > > This looks like a logical change, the code today does not do that
> > > anywhere, right?
> > 
> > `register_earlycon` only gets executed once.
> > Thus early_console_dev.port->uartclk is always 0.
> > Since all current callers in this patch series use `setup_earlycon`,
> > the uart_clk_freq parameter is also always 0.
> > 
> > So in case of behaviour/logic, this line does nothing without the 2.
> > patch.
> 
> Then it belongs in the second patch, right?
No.

It does nothing, because the newly added function
`setup_earlycon_with_uartclk` isn't used yet (first caller in the
second patch).

Moving this line in the second patch, would make the
`setup_earlycon_with_uartclk` function obsolete.

Thanks
- Markus Probst

> 
> thanks,
> 
> greg k-h

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