mtd: nand: ecc-hamming: Let the software Hamming ECC engine be unselected

There is no reason to always embed the software Hamming ECC engine
implementation. By default it is (with raw NAND), but we can let the
user decide.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200929230124.31491-19-miquel.raynal@bootlin.com

drivers/mtd/nand/Kconfig

@@ -16,7 +16,16 @@ config MTD_NAND_ECC
        depends on MTD_NAND_CORE
 
 config MTD_NAND_ECC_SW_HAMMING
-	bool
+	bool "Software Hamming ECC engine"
+	default y if MTD_RAW_NAND
+	select MTD_NAND_ECC
+	help
+	  This enables support for software Hamming error
+	  correction. This correction can correct up to 1 bit error
+	  per chunk and detect up to 2 bit errors. While it used to be
+	  widely used with old parts, newer NAND chips usually require
+	  more strength correction and in this case BCH or RS will be
+	  preferred.
 
 config MTD_NAND_ECC_SW_HAMMING_SMC
 	bool "NAND ECC Smart Media byte order"

drivers/mtd/nand/raw/Kconfig

@@ -3,7 +3,6 @@ menuconfig MTD_RAW_NAND
 	tristate "Raw/Parallel NAND Device Support"
 	select MTD_NAND_CORE
 	select MTD_NAND_ECC
-	select MTD_NAND_ECC_SW_HAMMING
 	help
 	  This enables support for accessing all type of raw/parallel
 	  NAND flash devices. For further information see
@@ -72,6 +71,7 @@ config MTD_NAND_AU1550
 config MTD_NAND_NDFC
 	tristate "IBM/MCC 4xx NAND controller"
 	depends on 4xx
+	select MTD_NAND_ECC_SW_HAMMING
 	select MTD_NAND_ECC_SW_HAMMING_SMC
 	help
 	  NDFC Nand Flash Controllers are integrated in IBM/AMCC's 4xx SoCs

include/linux/mtd/nand-ecc-sw-hamming.h

@@ -32,6 +32,8 @@ struct nand_ecc_sw_hamming_conf {
 	unsigned int sm_order;
 };
 
+#if IS_ENABLED(CONFIG_MTD_NAND_ECC_SW_HAMMING)
+
 int ecc_sw_hamming_calculate(const unsigned char *buf, unsigned int step_size,
 			     unsigned char *code, bool sm_order);
 int nand_ecc_sw_hamming_calculate(struct nand_device *nand,
@@ -44,4 +46,38 @@ int nand_ecc_sw_hamming_correct(struct nand_device *nand, unsigned char *buf,
 				unsigned char *read_ecc,
 				unsigned char *calc_ecc);
 
+#else /* !CONFIG_MTD_NAND_ECC_SW_HAMMING */
+
+static inline int ecc_sw_hamming_calculate(const unsigned char *buf,
+					   unsigned int step_size,
+					   unsigned char *code, bool sm_order)
+{
+	return -ENOTSUPP;
+}
+
+static inline int nand_ecc_sw_hamming_calculate(struct nand_device *nand,
+						const unsigned char *buf,
+						unsigned char *code)
+{
+	return -ENOTSUPP;
+}
+
+static inline int ecc_sw_hamming_correct(unsigned char *buf,
+					 unsigned char *read_ecc,
+					 unsigned char *calc_ecc,
+					 unsigned int step_size, bool sm_order)
+{
+	return -ENOTSUPP;
+}
+
+static inline int nand_ecc_sw_hamming_correct(struct nand_device *nand,
+					      unsigned char *buf,
+					      unsigned char *read_ecc,
+					      unsigned char *calc_ecc)
+{
+	return -ENOTSUPP;
+}
+
+#endif /* CONFIG_MTD_NAND_ECC_SW_HAMMING */
+
 #endif /* __MTD_NAND_ECC_SW_HAMMING_H__ */