net: sfp: initialize sfp->i2c_block_size at sfp allocation

sfp->i2c_block_size is initialized at SFP module insertion in
sfp_sm_mod_probe(). Because of that, if SFP module was never inserted
since boot, sfp_read() call will lead to zero-length I2C read attempt,
and not all I2C controllers are happy with zero-length reads.

One way to issue sfp_read() on empty SFP cage is to execute ethtool -m.
If SFP module was never plugged since boot, there will be a zero-length
I2C read attempt.

  # ethtool -m xge0
  i2c i2c-3: adapter quirk: no zero length (addr 0x0050, size 0, read)
  Cannot get Module EEPROM data: Operation not supported

If SFP module was plugged then removed at least once,
sfp->i2c_block_size will be initialized and ethtool -m will fail with
different exit code and without I2C error

  # ethtool -m xge0
  Cannot get Module EEPROM data: Remote I/O error

Fix this by initializing sfp->i2_block_size at struct sfp allocation
stage so no wild sfp_read() could issue zero-length I2C read.

Signed-off-by: Ivan Bornyakov <i.bornyakov@metrotek.ru>
Fixes: 0d035be ("net: sfp: VSOL V2801F / CarlitoxxPro CPGOS03-0490 v2.0 workaround")
Cc: stable@vger.kernel.org
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: Ivan Bornyakov

drivers/net/phy/sfp.c

@@ -210,6 +210,12 @@ static const enum gpiod_flags gpio_flags[] = {
 #define SFP_PHY_ADDR		22
 #define SFP_PHY_ADDR_ROLLBALL	17
 
+/* SFP_EEPROM_BLOCK_SIZE is the size of data chunk to read the EEPROM
+ * at a time. Some SFP modules and also some Linux I2C drivers do not like
+ * reads longer than 16 bytes.
+ */
+#define SFP_EEPROM_BLOCK_SIZE	16
+
 struct sff_data {
 	unsigned int gpios;
 	bool (*module_supported)(const struct sfp_eeprom_id *id);
@@ -1929,11 +1935,7 @@ static int sfp_sm_mod_probe(struct sfp *sfp, bool report)
 	u8 check;
 	int ret;
 
-	/* Some SFP modules and also some Linux I2C drivers do not like reads
-	 * longer than 16 bytes, so read the EEPROM in chunks of 16 bytes at
-	 * a time.
-	 */
-	sfp->i2c_block_size = 16;
+	sfp->i2c_block_size = SFP_EEPROM_BLOCK_SIZE;
 
 	ret = sfp_read(sfp, false, 0, &id.base, sizeof(id.base));
 	if (ret < 0) {
@@ -2621,6 +2623,7 @@ static struct sfp *sfp_alloc(struct device *dev)
 		return ERR_PTR(-ENOMEM);
 
 	sfp->dev = dev;
+	sfp->i2c_block_size = SFP_EEPROM_BLOCK_SIZE;
 
 	mutex_init(&sfp->sm_mutex);
 	mutex_init(&sfp->st_mutex);