[PATCH] net/nfp: update how MAX MTU is read

[Walter Heymans <walter.heymans@corigine.com>]

The 'max_rx_pktlen' value was previously read from hardware, which was
set by the running firmware. This caused confusion due to different
meanings of 'MAX_MTU'. This patch updates the 'max_rx_pktlen' to the
maximum value that the NFP NIC can support. The 'max_mtu' value that is
read from hardware, is assigned to the 'dev_info->max_mtu' variable.

If more layer 2 metadata must be used, the firmware can be updated to
report a smaller 'max_mtu' value.

The constant defined for NFP_FRAME_SIZE_MAX is derived for the maximum
supported buffer size of 10240, minus 136 bytes that is reserved by the
hardware and another 56 bytes reserved for expansion in firmware. This
results in a usable maximum packet length of 10048 bytes.

Signed-off-by: Walter Heymans <walter.heymans@corigine.com>
Signed-off-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Reviewed-by: Chaoyong He <chaoyong.he@corigine.com>
Reviewed-by: Richard Donkin <richard.donkin@corigine.com>
---
 drivers/net/nfp/nfp_common.c | 11 ++++++++++-
 drivers/net/nfp/nfp_common.h |  3 +++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/net/nfp/nfp_common.c b/drivers/net/nfp/nfp_common.c
index b26770dbfb..52fbda1a79 100644
--- a/drivers/net/nfp/nfp_common.c
+++ b/drivers/net/nfp/nfp_common.c
@@ -692,7 +692,16 @@ nfp_net_infos_get(struct rte_eth_dev *dev, struct rte_eth_dev_info *dev_info)
 	dev_info->max_rx_queues = (uint16_t)hw->max_rx_queues;
 	dev_info->max_tx_queues = (uint16_t)hw->max_tx_queues;
 	dev_info->min_rx_bufsize = RTE_ETHER_MIN_MTU;
-	dev_info->max_rx_pktlen = hw->max_mtu;
+	/*
+	 * The maximum rx packet length (max_rx_pktlen) is set to the
+	 * maximum supported frame size that the NFP can handle. This
+	 * includes layer 2 headers, CRC and other metadata that can
+	 * optionally be used.
+	 * The maximum layer 3 MTU (max_mtu) is read from hardware,
+	 * which was set by the firmware loaded onto the card.
+	 */
+	dev_info->max_rx_pktlen = NFP_FRAME_SIZE_MAX;
+	dev_info->max_mtu = hw->max_mtu;
 	/* Next should change when PF support is implemented */
 	dev_info->max_mac_addrs = 1;
 
diff --git a/drivers/net/nfp/nfp_common.h b/drivers/net/nfp/nfp_common.h
index 8b35fa119c..8db5ec23f8 100644
--- a/drivers/net/nfp/nfp_common.h
+++ b/drivers/net/nfp/nfp_common.h
@@ -98,6 +98,9 @@ struct nfp_net_adapter;
 /* Number of supported physical ports */
 #define NFP_MAX_PHYPORTS	12
 
+/* Maximum supported NFP frame size (MTU + layer 2 headers) */
+#define NFP_FRAME_SIZE_MAX	10048
+
 #include <linux/types.h>
 #include <rte_io.h>
 
-- 
2.25.1