net: sfp: read eeprom in maximum 16 byte increments

Some SFP modules do not like reads longer than 16 bytes, so read the
EEPROM in chunks of 16 bytes at a time.  This behaviour is not specified
in the SFP MSAs, which specifies:

 "The serial interface uses the 2-wire serial CMOS E2PROM protocol
  defined for the ATMEL AT24C01A/02/04 family of components."

and

 "As long as the SFP+ receives an acknowledge, it shall serially clock
  out sequential data words. The sequence is terminated when the host
  responds with a NACK and a STOP instead of an acknowledge."

We must avoid breaking a read across a 16-bit quantity in the diagnostic
page, thankfully all 16-bit quantities in that page are naturally
aligned.
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/sfp.c

@@ -281,6 +281,7 @@ static int sfp_i2c_read(struct sfp *sfp, bool a2, u8 dev_addr, void *buf,
 {
 	struct i2c_msg msgs[2];
 	u8 bus_addr = a2 ? 0x51 : 0x50;
+	size_t this_len;
 	int ret;
 
 	msgs[0].addr = bus_addr;
@@ -292,11 +293,26 @@ static int sfp_i2c_read(struct sfp *sfp, bool a2, u8 dev_addr, void *buf,
 	msgs[1].len = len;
 	msgs[1].buf = buf;
 
+	while (len) {
+		this_len = len;
+		if (this_len > 16)
+			this_len = 16;
+
+		msgs[1].len = this_len;
+
 		ret = i2c_transfer(sfp->i2c, msgs, ARRAY_SIZE(msgs));
 		if (ret < 0)
 			return ret;
 
-	return ret == ARRAY_SIZE(msgs) ? len : 0;
+		if (ret != ARRAY_SIZE(msgs))
+			break;
+
+		msgs[1].buf += this_len;
+		dev_addr += this_len;
+		len -= this_len;
+	}
+
+	return msgs[1].buf - (u8 *)buf;
 }
 
 static int sfp_i2c_write(struct sfp *sfp, bool a2, u8 dev_addr, void *buf,