ACPI: Add acpi=nospcr to disable ACPI SPCR as default console on ARM64

For varying privacy and security reasons, sometimes we would like to
completely silence the _serial_ console, and only enable it when needed.

But there are many existing systems that depend on this _serial_ console,
so add acpi=nospcr to disable console in ACPI SPCR table as default
_serial_ console.
Signed-off-by: Liu Wei <liuwei09@cestc.cn>
Suggested-by: Prarit Bhargava <prarit@redhat.com>
Suggested-by: Will Deacon <will@kernel.org>
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Reviewed-by: Prarit Bhargava <prarit@redhat.com>
Link: https://lore.kernel.org/r/20240625030504.58025-1-liuwei09@cestc.cnSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -12,7 +12,7 @@
 	acpi=		[HW,ACPI,X86,ARM64,RISCV64,EARLY]
 			Advanced Configuration and Power Interface
 			Format: { force | on | off | strict | noirq | rsdt |
-				  copy_dsdt }
+				  copy_dsdt | nospcr }
 			force -- enable ACPI if default was off
 			on -- enable ACPI but allow fallback to DT [arm64,riscv64]
 			off -- disable ACPI if default was on
@@ -21,8 +21,12 @@
 				strictly ACPI specification compliant.
 			rsdt -- prefer RSDT over (default) XSDT
 			copy_dsdt -- copy DSDT to memory
-			For ARM64 and RISCV64, ONLY "acpi=off", "acpi=on" or
-			"acpi=force" are available
+			nospcr -- disable console in ACPI SPCR table as
+				default _serial_ console on ARM64
+			For ARM64, ONLY "acpi=off", "acpi=on", "acpi=force" or
+			"acpi=nospcr" are available
+			For RISCV64, ONLY "acpi=off", "acpi=on" or "acpi=force"
+			are available
 
 			See also Documentation/power/runtime_pm.rst, pci=noacpi
 

arch/arm64/kernel/acpi.c

@@ -45,6 +45,7 @@ EXPORT_SYMBOL(acpi_pci_disabled);
 static bool param_acpi_off __initdata;
 static bool param_acpi_on __initdata;
 static bool param_acpi_force __initdata;
+static bool param_acpi_nospcr __initdata;
 
 static int __init parse_acpi(char *arg)
 {
@@ -58,6 +59,8 @@ static int __init parse_acpi(char *arg)
 		param_acpi_on = true;
 	else if (strcmp(arg, "force") == 0) /* force ACPI to be enabled */
 		param_acpi_force = true;
+	else if (strcmp(arg, "nospcr") == 0) /* disable SPCR as default console */
+		param_acpi_nospcr = true;
 	else
 		return -EINVAL;	/* Core will print when we return error */
 
@@ -237,7 +240,20 @@ void __init acpi_boot_table_init(void)
 			acpi_put_table(facs);
 		}
 #endif
-		acpi_parse_spcr(earlycon_acpi_spcr_enable, true);
+
+		/*
+		 * For varying privacy and security reasons, sometimes need
+		 * to completely silence the serial console output, and only
+		 * enable it when needed.
+		 * But there are many existing systems that depend on this
+		 * behaviour, use acpi=nospcr to disable console in ACPI SPCR
+		 * table as default serial console.
+		 */
+		acpi_parse_spcr(earlycon_acpi_spcr_enable,
+			!param_acpi_nospcr);
+		pr_info("Use ACPI SPCR as default console: %s\n",
+				param_acpi_nospcr ? "No" : "Yes");
+
 		if (IS_ENABLED(CONFIG_ACPI_BGRT))
 			acpi_table_parse(ACPI_SIG_BGRT, acpi_parse_bgrt);
 	}