Re: [PATCH net v2 5/6] net: stmmac: size the RX buffers from the frame length, not the MTU

When picking the buffsize to use based on the MTU, we shouldn't check
only the MTU value, but also :
 - ETH_HLEN for the L2 header,
 - up to 2 VLAN tags,
 - the FCS,
 - the NET_IP_ALIGN

The default bufsize is 1536 bytes, which is enough to contain all the
above so this hasn't surfaced before, but the addition of NET_IP_ALIGN
to the start of buffer address tripped the Jumbo selftest, leading to
this discovery.

With that, we don't need the '>=' checks on the buffer len, we can use
more consistent comparison operators in stmmac_set_bfsize.

Fixes: 286a83721720 ("stmmac: add CHAINED descriptor mode support 
(V4)")
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
---
 .../net/ethernet/stmicro/stmmac/stmmac_main.c | 20 ++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c 

b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 5fe7e95fdd34..3664a298574a 100644

--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -1536,17 +1536,17 @@ static unsigned int stmmac_rx_offset(struct 

stmmac_priv *priv)
 	return NET_SKB_PAD + NET_IP_ALIGN;
 }

-static int stmmac_set_bfsize(int mtu)
+static int stmmac_set_bfsize(int len)
 {
 	int ret;

-	if (mtu >= BUF_SIZE_8KiB)
+	if (len > BUF_SIZE_8KiB)
 		ret = BUF_SIZE_16KiB;

-	else if (mtu >= BUF_SIZE_4KiB)
+	else if (len > BUF_SIZE_4KiB)
 		ret = BUF_SIZE_8KiB;
-	else if (mtu >= BUF_SIZE_2KiB)
+	else if (len > BUF_SIZE_2KiB)
 		ret = BUF_SIZE_4KiB;
-	else if (mtu > DEFAULT_BUFSIZE)
+	else if (len > DEFAULT_BUFSIZE)
 		ret = BUF_SIZE_2KiB;
 	else
 		ret = DEFAULT_BUFSIZE;

@@ -4063,7 +4063,7 @@ static struct stmmac_dma_conf *
 stmmac_setup_dma_desc(struct stmmac_priv *priv, unsigned int mtu)
 {
 	struct stmmac_dma_conf *dma_conf;
-	int bfsize, ret;
+	int bfsize, len, ret;
 	u8 chan;

 	dma_conf = kzalloc_obj(*dma_conf);

@@ -4073,13 +4073,15 @@ stmmac_setup_dma_desc(struct stmmac_priv *priv, 

unsigned int mtu)
 		return ERR_PTR(-ENOMEM);
 	}

-	/* Returns 0 or BUF_SIZE_16KiB if mtu > 8KiB and dwmac4 or ring mode 
*/
-	bfsize = stmmac_set_16kib_bfsize(priv, mtu);
+	len = mtu + ETH_HLEN + 2 * VLAN_HLEN + ETH_FCS_LEN + NET_IP_ALIGN;
+
+	/* Returns 0 or BUF_SIZE_16KiB if len > 8KiB and dwmac4 or ring mode 
*/
+	bfsize = stmmac_set_16kib_bfsize(priv, len);
 	if (bfsize < 0)
 		bfsize = 0;

 	if (bfsize < BUF_SIZE_16KiB)
-		bfsize = stmmac_set_bfsize(mtu);
+		bfsize = stmmac_set_bfsize(len);

 	dma_conf->dma_buf_sz = bfsize;
 	/* Chose the tx/rx size from the already defined one in the