acpi/arm64: ignore 5.1 FADTs that are reported as 5.0

Some Qualcomm Snapdragon based laptops built to run Microsoft Windows
are clearly ACPI 5.1 based, given that that is the first ACPI revision
that supports ARM, and introduced the FADT 'arm_boot_flags' field,
which has a non-zero field on those systems.

So in these cases, infer from the ARM boot flags that the FADT must be
5.1 or later, and treat it as 5.1.

Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Graeme Gregory <graeme.gregory@linaro.org>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/acpi.c

@@ -155,11 +155,15 @@ static int __init acpi_fadt_sanity_check(void)
 	 */
 	if (table->revision < 5 ||
 	   (table->revision == 5 && fadt->minor_revision < 1)) {
-		pr_err("Unsupported FADT revision %d.%d, should be 5.1+\n",
+		pr_err(FW_BUG "Unsupported FADT revision %d.%d, should be 5.1+\n",
 		       table->revision, fadt->minor_revision);
+
+		if (!fadt->arm_boot_flags) {
 			ret = -EINVAL;
 			goto out;
 		}
+		pr_err("FADT has ARM boot flags set, assuming 5.1\n");
+	}
 
 	if (!(fadt->flags & ACPI_FADT_HW_REDUCED)) {
 		pr_err("FADT not ACPI hardware reduced compliant\n");