[PATCH v4] ACPI: SPCR: Support UART clock frequency field

[Markus Probst <markus.probst@posteo.de>]

Prior to this patch, we assume the uart clock frequency is 1843200 Hz in
the early console. This behaviour results in garbage console output if the
actual uart clock frequency differs.

The UART Clock Frequency field was added in the Microsoft Serial Port
Console Redirection (SPCR) specification revision 1.08. If present, use it
to configure the serial port with the correct uart clock frequency.
Fallback to old behaviour if missing.

Add function `setup_earlycon_with_uartclk` to set uart clock frequency
while still allowing to reuse the same console string with
`add_preferred_console`.

Link: https://learn.microsoft.com/en-us/windows-hardware/drivers/serports/serial-port-console-redirection-table
Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
Changes in v4:
- merge patches together
- rewrite commit message
- Link to v3: https://patch.msgid.link/20260615-acpi_spcr-v3-0-9a59ebad74ea@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
---
 drivers/acpi/spcr.c           |  3 ++-
 drivers/tty/serial/earlycon.c | 17 ++++++++++++-----
 include/linux/serial_core.h   | 11 +++++++++--
 3 files changed, 23 insertions(+), 8 deletions(-)

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);
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 bdc214386e4a..9b2bcd4295a9 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -1099,11 +1099,18 @@ 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)

---
base-commit: 237a1c39e8dfd3e1c6f1f023eea37a48ec04cc63
change-id: 20260430-acpi_spcr-61902fd923f2