e1000e: ignore uninitialized checksum word on tgp

As described by Vitaly Lifshits:

> Starting from Tiger Lake, LAN NVM is locked for writes by SW, so the
> driver cannot perform checksum validation and correction. This means
> that all NVM images must leave the factory with correct checksum and
> checksum valid bit set.

Unfortunately some systems have left the factory with an uninitialized
value of 0xFFFF at register address 0x3F (checksum word location).
So on Tiger Lake platform we ignore the computed checksum when such
condition is encountered.

Signed-off-by: Jacek Kowalski <[email protected]>
Tested-by: Vlad URSU <[email protected]>
Fixes: 4051f68 ("e1000e: Do not take care about recovery NVM checksum")
Cc: [email protected]
Reviewed-by: Simon Horman <[email protected]>
Reviewed-by: Vitaly Lifshits <[email protected]>
Tested-by: Mor Bar-Gabay <[email protected]>
Signed-off-by: Tony Nguyen <[email protected]>

Author: jacekkow

drivers/net/ethernet/intel/e1000e/defines.h

@@ -638,6 +638,9 @@
 /* For checksumming, the sum of all words in the NVM should equal 0xBABA. */
 #define NVM_SUM                    0xBABA
 
+/* Uninitialized ("empty") checksum word value */
+#define NVM_CHECKSUM_UNINITIALIZED 0xFFFF
+
 /* PBA (printed board assembly) number words */
 #define NVM_PBA_OFFSET_0           8
 #define NVM_PBA_OFFSET_1           9

drivers/net/ethernet/intel/e1000e/nvm.c

@@ -558,6 +558,12 @@ s32 e1000e_validate_nvm_checksum_generic(struct e1000_hw *hw)
 		checksum += nvm_data;
 	}
 
+	if (hw->mac.type == e1000_pch_tgp &&
+	    nvm_data == NVM_CHECKSUM_UNINITIALIZED) {
+		e_dbg("Uninitialized NVM Checksum on TGP platform - ignoring\n");
+		return 0;
+	}
+
 	if (checksum != (u16)NVM_SUM) {
 		e_dbg("NVM Checksum Invalid\n");
 		return -E1000_ERR_NVM;