efivar/ssdt: Don't iterate over EFI vars if no SSDT override was specified

The kernel command line option efivar_ssdt= allows the name to be
specified of an EFI variable containing an ACPI SSDT table that should
be loaded into memory by the OS, and treated as if it was provided by
the firmware.

Currently, that code will always iterate over the EFI variables and
compare each name with the provided name, even if the command line
option wasn't set to begin with.

So bail early when no variable name was provided. This works around a
boot regression on the 2012 Mac Pro, as reported by Scott.

Tested-by: Scott Talbert <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>
Cc: <[email protected]> # v4.9+
Cc: Ben Dooks <[email protected]>
Cc: Dave Young <[email protected]>
Cc: Jarkko Sakkinen <[email protected]>
Cc: Jerry Snitselaar <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Lukas Wunner <[email protected]>
Cc: Lyude Paul <[email protected]>
Cc: Matthew Garrett <[email protected]>
Cc: Octavian Purdila <[email protected]>
Cc: Peter Jones <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Fixes: 475fb4e ("efi / ACPI: load SSTDs from EFI variables")
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: Ard Biesheuvel

drivers/firmware/efi/efi.c

@@ -267,6 +267,9 @@ static __init int efivar_ssdt_load(void)
 	void *data;
 	int ret;
 
+	if (!efivar_ssdt[0])
+		return 0;
+
 	ret = efivar_init(efivar_ssdt_iter, &entries, true, &entries);
 
 	list_for_each_entry_safe(entry, aux, &entries, list) {