arm64: acpi: add acpi=on cmdline option to prefer ACPI boot over DT

If both ACPI and DT platform descriptions are available, and the
kernel was configured at build time to support both flavours, the
default policy is to prefer DT over ACPI, and preferring ACPI over
DT while still allowing DT as a fallback is not possible.

Since some enterprise features (such as RAS) depend on ACPI, it may
be desirable for, e.g., distro installers to prefer ACPI boot but
fall back to DT rather than failing completely if no ACPI tables are
available.

So introduce the 'acpi=on' kernel command line parameter for arm64,
which signifies that ACPI should be used if available, and DT should
only be used as a fallback.
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Documentation/kernel-parameters.txt

@@ -167,16 +167,18 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	acpi=		[HW,ACPI,X86,ARM64]
 			Advanced Configuration and Power Interface
-			Format: { force | off | strict | noirq | rsdt |
+			Format: { force | on | off | strict | noirq | rsdt |
 				  copy_dsdt }
 			force -- enable ACPI if default was off
+			on -- enable ACPI but allow fallback to DT [arm64]
 			off -- disable ACPI if default was on
 			noirq -- do not use ACPI for IRQ routing
 			strict -- Be less tolerant of platforms that are not
 				strictly ACPI specification compliant.
 			rsdt -- prefer RSDT over (default) XSDT
 			copy_dsdt -- copy DSDT to memory
-			For ARM64, ONLY "acpi=off" or "acpi=force" are available
+			For ARM64, ONLY "acpi=off", "acpi=on" or "acpi=force"
+			are available
 
 			See also Documentation/power/runtime_pm.txt, pci=noacpi
 

arch/arm64/kernel/acpi.c

@@ -42,6 +42,7 @@ int acpi_pci_disabled = 1;	/* skip ACPI PCI scan and IRQ initialization */
 EXPORT_SYMBOL(acpi_pci_disabled);
 
 static bool param_acpi_off __initdata;
+static bool param_acpi_on __initdata;
 static bool param_acpi_force __initdata;
 
 static int __init parse_acpi(char *arg)
@@ -52,6 +53,8 @@ static int __init parse_acpi(char *arg)
 	/* "acpi=off" disables both ACPI table parsing and interpreter */
 	if (strcmp(arg, "off") == 0)
 		param_acpi_off = true;
+	else if (strcmp(arg, "on") == 0) /* prefer ACPI over DT */
+		param_acpi_on = true;
 	else if (strcmp(arg, "force") == 0) /* force ACPI to be enabled */
 		param_acpi_force = true;
 	else
@@ -198,10 +201,11 @@ void __init acpi_boot_table_init(void)
 	 * - ACPI has been disabled explicitly (acpi=off), or
 	 * - the device tree is not empty (it has more than just a /chosen node,
 	 *   and a /hypervisor node when running on Xen)
-	 *   and ACPI has not been force enabled (acpi=force)
+	 *   and ACPI has not been [force] enabled (acpi=on|force)
 	 */
 	if (param_acpi_off ||
-	    (!param_acpi_force && of_scan_flat_dt(dt_scan_depth1_nodes, NULL)))
+	    (!param_acpi_on && !param_acpi_force &&
+	     of_scan_flat_dt(dt_scan_depth1_nodes, NULL)))
 		return;
 
 	/*