iommu/vt-d: Do not falsely log intel_iommu is unsupported kernel option

Handling of intel_iommu kernel command line option should return "true" to
indicate option is valid and so avoid logging it as unknown by the core
parsing code.

Also log unknown sub-options at the notice level to let user know of
potential typos or similar.
Reported-by: Eero Tamminen <eero.t.tamminen@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://lore.kernel.org/r/20210831112947.310080-1-tvrtko.ursulin@linux.intel.comSigned-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20211014053839.727419-2-baolu.lu@linux.intel.comSigned-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/intel/iommu.c

@@ -412,6 +412,7 @@ static int __init intel_iommu_setup(char *str)
 {
 	if (!str)
 		return -EINVAL;
+
 	while (*str) {
 		if (!strncmp(str, "on", 2)) {
 			dmar_disabled = 0;
@@ -441,13 +442,16 @@ static int __init intel_iommu_setup(char *str)
 		} else if (!strncmp(str, "tboot_noforce", 13)) {
 			pr_info("Intel-IOMMU: not forcing on after tboot. This could expose security risk for tboot\n");
 			intel_iommu_tboot_noforce = 1;
+		} else {
+			pr_notice("Unknown option - '%s'\n", str);
 		}
 
 		str += strcspn(str, ",");
 		while (*str == ',')
 			str++;
 	}
-	return 0;
+
+	return 1;
 }
 __setup("intel_iommu=", intel_iommu_setup);