NFC: digital: Add NFC-DEP Initiator-side NACK Support

When an NFC-DEP Initiator receives a frame with
an incorrect CRC or with a parity error, and the
frame is at least 4 bytes long, its supposed to
send a NACK to the Target.  The Initiator can
send up to 'N(retry,nack)' consecutive NACKs
where 2 <= 'N(retry,nack)' <= 5.  When the limit
is exceeded, a PROTOCOL EXCEPTION is raised.
Any other type of transmission error is to be
ignored and the Initiator should continue
waiting for a new frame.  This is described
in section 14.12.5.4 of the NFC Digital Protocol
Spec.

The digital layer's NFC-DEP code doesn't implement
any of this so add it.  This support diverges from
the spec in two significant ways:

a) NACKs will be sent for ANY error reported by the
   driver except a timeout.  This is done because
   there is currently no way for the digital layer
   to distinguish a CRC or parity error from any
   other type of error reported by the driver.

b) All other errors will cause a PROTOCOL EXCEPTION
   even frames with CRC errors that are less than 4
   bytes.

The value chosen for 'N(retry,nack)' is 2.

Targets do not send NACK PDUs.

Reviewed-by: Thierry Escande <thierry.escande@linux.intel.com>
Tested-by: Thierry Escande <thierry.escande@linux.intel.com>
Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

include/net/nfc/digital.h

@@ -233,6 +233,8 @@ struct nfc_digital_dev {
 	struct sk_buff *chaining_skb;
 	struct digital_data_exch *data_exch;
 
+	int nack_count;
+
 	u16 target_fsc;
 
 	int (*skb_check_crc)(struct sk_buff *skb);