Add a new rx_frame_size member in the fec_enet_private structure to
decouple frame size configuration from max_buf_size. This allows more
precise control over RX frame length settings. It is particularly useful
for Jumbo frame support because the RX frame size may possible larger than
the allocated RX buffer.

Configure TRUNC_FL (Frame Truncation Length) based on the RX buffer size.
Frames exceeding this limit will be treated as error packets and dropped.

Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
---
 drivers/net/ethernet/freescale/fec.h      | 1 +
 drivers/net/ethernet/freescale/fec_main.c | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/freescale/fec.h b/drivers/net/ethernet/freescale/fec.h
index 47317346b2f3..f1032a11aa76 100644
--- a/drivers/net/ethernet/freescale/fec.h
+++ b/drivers/net/ethernet/freescale/fec.h
@@ -621,6 +621,7 @@ struct fec_enet_private {
 	unsigned int total_rx_ring_size;
 	unsigned int max_buf_size;
 	unsigned int pagepool_order;
+	unsigned int rx_frame_size;
 
 	struct	platform_device *pdev;
 
diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
index f046d32a62fb..3f25db23d0de 100644
--- a/drivers/net/ethernet/freescale/fec_main.c
+++ b/drivers/net/ethernet/freescale/fec_main.c
@@ -1191,7 +1191,7 @@ fec_restart(struct net_device *ndev)
 		else
 			val &= ~FEC_RACC_OPTIONS;
 		writel(val, fep->hwp + FEC_RACC);
-		writel(fep->max_buf_size, fep->hwp + FEC_FTRL);
+		writel(fep->rx_frame_size, fep->hwp + FEC_FTRL);
 	}
 #endif
 
@@ -4560,6 +4560,7 @@ fec_probe(struct platform_device *pdev)
 	pinctrl_pm_select_sleep_state(&pdev->dev);
 
 	fep->pagepool_order = 0;
+	fep->rx_frame_size = FEC_ENET_RX_FRSIZE;
 	fep->max_buf_size = PKT_MAXBUF_SIZE;
 	ndev->max_mtu = fep->max_buf_size - ETH_HLEN - ETH_FCS_LEN;
 
-- 
2.43.0

On Sat, Aug 23, 2025 at 02:01:08PM -0500, Shenwei Wang wrote:
> Add a new rx_frame_size member in the fec_enet_private structure to
> decouple frame size configuration from max_buf_size. This allows more
> precise control over RX frame length settings. It is particularly useful
> for Jumbo frame support because the RX frame size may possible larger than
> the allocated RX buffer.

Please could you extend that a little. What happens if the received
frame is bigger than the buffer? Does the hardware fragment it over
two buffers?

> 
> Configure TRUNC_FL (Frame Truncation Length) based on the RX buffer size.
> Frames exceeding this limit will be treated as error packets and dropped.

This bit confuses me. You want to allow rx_frame_size to be bigger
than the buffer size, but you also want to discard frames bigger than
the buffer size?

    Andrew

---
pw-bot: cr