The RX buffers for GEM can have a maximum size of 16320 bytes
(0xff in the RXBS field of the DMACFG register means 255*64 =
16320 bytes).

The GEM IP has configurable maximum jumbo frame length that can go up to
16383. The actual value for this limit can be found in the
       "jumbo_max_length" field (bits 0..13) of the DCFG2 register.
Currently, the macb driver doesn't use the DCFG2 register when
determining the max MTU, instead an hardcoded value (jumbo_max_len in
struct macb_config) is used for each platform. Right now the maximum
value for jumbo_max_len is 10240 (0x2800).

GEM uses one buffer per packet which means that one buffer must allow
room for the max MTU plus L2 encapsulation and alignment. This is a
limitation of the driver.

This commit adds a limit to max_mtu and rx_buffer_size so that the RXBS
field can never overflow when a large MTU is used.

With this commit, it is now possible to add new platforms with a
jumbo_max_len of 16383 so that the hardware properties of each IP can be
properly captured in struct macb_config.

Signed-off-by: Charles Perry <charles.perry@microchip.com>
Reviewed-by: Simon Horman <horms@kernel.org>
---
 drivers/net/ethernet/cadence/macb_main.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index dc661e5a0769..96e15f58e173 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -50,6 +50,7 @@ struct sifive_fu540_macb_mgmt {
 
 #define MACB_RX_BUFFER_SIZE	128
 #define RX_BUFFER_MULTIPLE	64  /* bytes */
+#define RX_BUFFER_MAX		(0xFF * RX_BUFFER_MULTIPLE) /* 16320 bytes */
 
 #define DEFAULT_RX_RING_SIZE	512 /* must be power of 2 */
 #define MIN_RX_RING_SIZE	64
@@ -2393,7 +2394,7 @@ static void macb_init_rx_buffer_size(struct macb *bp, size_t size)
 	if (!macb_is_gem(bp)) {
 		bp->rx_buffer_size = MACB_RX_BUFFER_SIZE;
 	} else {
-		bp->rx_buffer_size = size;
+		bp->rx_buffer_size = MIN(size, RX_BUFFER_MAX);
 
 		if (bp->rx_buffer_size % RX_BUFFER_MULTIPLE) {
 			netdev_dbg(bp->dev,
@@ -5588,7 +5589,8 @@ static int macb_probe(struct platform_device *pdev)
 	/* MTU range: 68 - 1518 or 10240 */
 	dev->min_mtu = GEM_MTU_MIN_SIZE;
 	if ((bp->caps & MACB_CAPS_JUMBO) && bp->jumbo_max_len)
-		dev->max_mtu = bp->jumbo_max_len - ETH_HLEN - ETH_FCS_LEN;
+		dev->max_mtu = MIN(bp->jumbo_max_len, RX_BUFFER_MAX) -
+				ETH_HLEN - ETH_FCS_LEN;
 	else
 		dev->max_mtu = 1536 - ETH_HLEN - ETH_FCS_LEN;
 
-- 
2.47.3