dmi_scan: add comments on dmi_present() and the loop in dmi_scan_machine()

My previous refactoring in commit 79bae42d51a5 ("dmi_scan: refactor
dmi_scan_machine(), {smbios,dmi}_present()") resulted in slightly tricky
code (though I think it's more elegant). Explain what it's doing.

Signed-off-by: Ben Hutchings <[e-mail]>
Cc: Zhenzhong Duan <[e-mail]>
Cc: Jean Delvare <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/firmware/dmi_scan.c

···
 			    dmi_get_system_info(DMI_BIOS_DATE));
 }
 
+/*
+ * Check for DMI/SMBIOS headers in the system firmware image.  Any
+ * SMBIOS header must start 16 bytes before the DMI header, so take a
+ * 32 byte buffer and check for DMI at offset 16 and SMBIOS at offset
+ * 0.  If the DMI header is present, set dmi_ver accordingly (SMBIOS
+ * takes precedence) and return 0.  Otherwise return 1.
+ */
 static int __init dmi_present(const u8 *buf)
 {
 	int smbios_ver;
···
 		if (p == NULL)
 			goto error;
 
+		/*
+		 * Iterate over all possible DMI header addresses q.
+		 * Maintain the 32 bytes around q in buf.  On the
+		 * first iteration, substitute zero for the
+		 * out-of-range bytes so there is no chance of falsely
+		 * detecting an SMBIOS header.
+		 */
 		memset(buf, 0, 16);
 		for (q = p; q < p + 0x10000; q += 16) {
 			memcpy_fromio(buf + 16, q, 16);