NFC: pn533: Add some polling entropy

By not always starting the polling loop from the same modulation, we
avoid entering infinite loops where devices exporting 2 targets (on 2
different modulations) get the same target activated over and over.
If this target is not readable (e.g. a wallet emulating a tag), we will
stay in an error loop for ever.
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/nfc/pn533.c

@@ -1913,6 +1913,7 @@ static int pn533_start_poll(struct nfc_dev *nfc_dev,
 			    u32 im_protocols, u32 tm_protocols)
 {
 	struct pn533 *dev = nfc_get_drvdata(nfc_dev);
+	u8 rand_mod;
 
 	nfc_dev_dbg(&dev->interface->dev,
 		    "%s: im protocols 0x%x tm protocols 0x%x",
@@ -1936,11 +1937,15 @@ static int pn533_start_poll(struct nfc_dev *nfc_dev,
 			tm_protocols = 0;
 	}
 
-	dev->poll_mod_curr = 0;
 	pn533_poll_create_mod_list(dev, im_protocols, tm_protocols);
 	dev->poll_protocols = im_protocols;
 	dev->listen_protocols = tm_protocols;
 
+	/* Do not always start polling from the same modulation */
+	get_random_bytes(&rand_mod, sizeof(rand_mod));
+	rand_mod %= dev->poll_mod_count;
+	dev->poll_mod_curr = rand_mod;
+
 	return pn533_send_poll_frame(dev);
 }