linux-kernel - [PATCH v2] x86/acpi: Add acpi=spcr to use SPCR-provided default console

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Message-ID: <20260117072827.355360-1-me@shenghaoyang.info>
Date: Sat, 17 Jan 2026 15:28:27 +0800
From: Shenghao Yang <me@...nghaoyang.info>
To: x86@...nel.org
Cc: Jonathan Corbet <corbet@....net>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Len Brown <lenb@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-acpi@...r.kernel.org,
	Shenghao Yang <me@...nghaoyang.info>
Subject: [PATCH v2] x86/acpi: Add acpi=spcr to use SPCR-provided default console

The SPCR provided console on x86 is only available as a boot
console when earlycon is provided on the kernel command line,
and will not be present in /proc/consoles.

While it's possible to retain the boot console with the
keep_bootcon parameter, that leaves the console using the less
efficient 8250_early driver.

Users wanting to use the firmware suggested console (to avoid
maintaining unique serial console parameters for different
server models in large fleets) with the conventional driver
have to parse the kernel log for the console parameters and
reinsert them.

[    0.005091] ACPI: SPCR 0x000000007FFB5000 000059 (v04 ALASKA A M I    01072009 INTL 20250404)
[    0.073387] ACPI: SPCR: console: uart,io,0x3f8,115200

In commit 0231d00082f6 ("ACPI: SPCR: Make SPCR available to x86") [1]
the SPCR console was only added as an option for earlycon but
not as an ordinary console so users don't see console output
changes.

So users can opt in to an automatic SPCR console, make ACPI
init add it if acpi=spcr is set.

[1]: https://lore.kernel.org/lkml/20180118150951.28964-1-prarit@redhat.com/

Signed-off-by: Shenghao Yang <me@...nghaoyang.info>
---
v2: Reworded commit with more detail.

[v1]: https://lore.kernel.org/lkml/20251228092222.130954-1-me@shenghaoyang.info/

 Documentation/admin-guide/kernel-parameters.txt | 2 ++
 arch/x86/kernel/acpi/boot.c                     | 7 ++++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 1058f2a6d6a8..9b65ec0b5e0a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -125,6 +125,8 @@ Kernel parameters
 			may result in duplicate corrected error reports.
 			nospcr -- disable console in ACPI SPCR table as
 				default _serial_ console on ARM64
+			spcr -- enable console in ACPI SPCR table as
+				default _serial_ console on X86
 			For ARM64, ONLY "acpi=off", "acpi=on", "acpi=force" or
 			"acpi=nospcr" are available
 			For RISCV64, ONLY "acpi=off", "acpi=on" or "acpi=force"
diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
index 9fa321a95eb3..83bbfa1d6f1f 100644
--- a/arch/x86/kernel/acpi/boot.c
+++ b/arch/x86/kernel/acpi/boot.c
@@ -47,6 +47,7 @@ EXPORT_SYMBOL(acpi_disabled);
 
 int acpi_noirq;				/* skip ACPI IRQ initialization */
 static int acpi_nobgrt;			/* skip ACPI BGRT */
+static int acpi_spcr_add __initdata;		/* add SPCR-provided console */
 int acpi_pci_disabled;		/* skip ACPI PCI scan and IRQ initialization */
 EXPORT_SYMBOL(acpi_pci_disabled);
 
@@ -1666,7 +1667,7 @@ int __init acpi_boot_init(void)
 		x86_init.pci.init = pci_acpi_init;
 
 	/* Do not enable ACPI SPCR console by default */
-	acpi_parse_spcr(earlycon_acpi_spcr_enable, false);
+	acpi_parse_spcr(earlycon_acpi_spcr_enable, acpi_spcr_add);
 	return 0;
 }
 
@@ -1703,6 +1704,10 @@ static int __init parse_acpi(char *arg)
 	/* "acpi=nocmcff" disables FF mode for corrected errors */
 	else if (strcmp(arg, "nocmcff") == 0) {
 		acpi_disable_cmcff = 1;
+	}
+	/* "acpi=spcr" adds the SPCR-provided console as a preferred one */
+	else if (strcmp(arg, "spcr") == 0) {
+		acpi_spcr_add = 1;
 	} else {
 		/* Core will printk when we return error. */
 		return -EINVAL;
-- 
2.52.0