Add comments about the validation.

svn path=/head/; revision=222967

sys/dev/atkbdc/atkbd.c

@@ -1097,6 +1097,15 @@ get_typematic(keyboard_t *kbd)
 	x86regs_t regs;
 	uint8_t *p;
 
+	/*
+	 * Traditional entry points of int 0x15 and 0x16 are fixed
+	 * and later BIOSes follow them.  (U)EFI CSM specification
+	 * also mandate these fixed entry points.
+	 *
+	 * Validate the entry points here before we proceed further.
+	 * It's known that some recent laptops does not have the
+	 * same entry point and hang on boot if we call it.
+	 */
 	if (x86bios_get_intr(0x15) != 0xf000f859 ||
 	    x86bios_get_intr(0x16) != 0xf000e82e)
 		return (ENODEV);