ethtool: cmis_cdb: Fix incorrect read / write length extension

The 'read_write_len_ext' field in 'struct ethtool_cmis_cdb_cmd_args'
stores the maximum number of bytes that can be read from or written to
the Local Payload (LPL) page in a single multi-byte access.

Cited commit started overwriting this field with the maximum number of
bytes that can be read from or written to the Extended Payload (LPL)
pages in a single multi-byte access. Transceiver modules that support
auto paging can advertise a number larger than 255 which is problematic
as 'read_write_len_ext' is a 'u8', resulting in the number getting
truncated and firmware flashing failing [1].

Fix by ignoring the maximum EPL access size as the kernel does not
currently support auto paging (even if the transceiver module does) and
will not try to read / write more than 128 bytes at once.

[1]
Transceiver module firmware flashing started for device enp177s0np0
Transceiver module firmware flashing in progress for device enp177s0np0
Progress: 0%
Transceiver module firmware flashing encountered an error for device enp177s0np0
Status message: Write FW block EPL command failed, LPL length is longer
	than CDB read write length extension allows.

Fixes: 9a3b0d0 ("net: ethtool: Add support for writing firmware blocks using EPL payload")
Reported-by: Damodharam Ammepalli <damodharam.ammepalli@broadcom.com>
Closes: https://lore.kernel.org/netdev/20250402183123.321036-3-michael.chan@broadcom.com/
Tested-by: Damodharam Ammepalli <damodharam.ammepalli@broadcom.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Damodharam Ammepalli <damodharam.ammepalli@broadcom.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20250409112440.365672-1-idosch@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/ethtool/cmis.h

@@ -101,7 +101,6 @@ struct ethtool_cmis_cdb_rpl {
 };
 
 u32 ethtool_cmis_get_max_lpl_size(u8 num_of_byte_octs);
-u32 ethtool_cmis_get_max_epl_size(u8 num_of_byte_octs);
 
 void ethtool_cmis_cdb_compose_args(struct ethtool_cmis_cdb_cmd_args *args,
 				   enum ethtool_cmis_cdb_cmd_id cmd, u8 *lpl,

net/ethtool/cmis_cdb.c

@@ -16,15 +16,6 @@ u32 ethtool_cmis_get_max_lpl_size(u8 num_of_byte_octs)
 	return 8 * (1 + min_t(u8, num_of_byte_octs, 15));
 }
 
-/* For accessing the EPL field on page 9Fh, the allowable length extension is
- * min(i, 255) byte octets where i specifies the allowable additional number of
- * byte octets in a READ or a WRITE.
- */
-u32 ethtool_cmis_get_max_epl_size(u8 num_of_byte_octs)
-{
-	return 8 * (1 + min_t(u8, num_of_byte_octs, 255));
-}
-
 void ethtool_cmis_cdb_compose_args(struct ethtool_cmis_cdb_cmd_args *args,
 				   enum ethtool_cmis_cdb_cmd_id cmd, u8 *lpl,
 				   u8 lpl_len, u8 *epl, u16 epl_len,
@@ -33,19 +24,16 @@ void ethtool_cmis_cdb_compose_args(struct ethtool_cmis_cdb_cmd_args *args,
 {
 	args->req.id = cpu_to_be16(cmd);
 	args->req.lpl_len = lpl_len;
-	if (lpl) {
+	if (lpl)
 		memcpy(args->req.payload, lpl, args->req.lpl_len);
-		args->read_write_len_ext =
-			ethtool_cmis_get_max_lpl_size(read_write_len_ext);
-	}
 	if (epl) {
 		args->req.epl_len = cpu_to_be16(epl_len);
 		args->req.epl = epl;
-		args->read_write_len_ext =
-			ethtool_cmis_get_max_epl_size(read_write_len_ext);
 	}
 
 	args->max_duration = max_duration;
+	args->read_write_len_ext =
+		ethtool_cmis_get_max_lpl_size(read_write_len_ext);
 	args->msleep_pre_rpl = msleep_pre_rpl;
 	args->rpl_exp_len = rpl_exp_len;
 	args->flags = flags;