[PATCH net-next] net: stmmac: improve ndev->max_mtu setup readability

[PATCH net-next] net: stmmac: improve ndev->max_mtu setup readability

[Russell King (Oracle)]

Improve the readibility of the code setting ndev->max_mtu. This depends
on the hardware specific maximum defined by the MAC core, and also a
platform provided maximum.

The code was originally checking that the platform specific maximum was
between ndev->min_mtu..MAC core maximum before reducing ndev->max_mtu,
otherwise if the platform specific maximum was less than ndev->min_mtu,
issuing a warning.

Re-order the code to handle the case where the platform specific max is
below ndev->min_mtu, which then means that the subsequent test is
simply reducing ndev->max_mtu.

Update the comment, and add a few blank lines to separate the blocks of
code.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
---
 .../net/ethernet/stmicro/stmmac/stmmac_main.c | 22 ++++++++++---------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index ccf383b355e7..eb4350193996 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -1392,9 +1392,9 @@ static unsigned int stmmac_rx_offset(struct stmmac_priv *priv)
 	return NET_SKB_PAD;
 }
 
-static int stmmac_set_bfsize(int mtu, int bufsize)
+static int stmmac_set_bfsize(int mtu)
 {
-	int ret = bufsize;
+	int ret;
 
 	if (mtu >= BUF_SIZE_8KiB)
 		ret = BUF_SIZE_16KiB;
@@ -3958,12 +3958,13 @@ 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);
 	if (bfsize < 0)
 		bfsize = 0;
 
 	if (bfsize < BUF_SIZE_16KiB)
-		bfsize = stmmac_set_bfsize(mtu, 0);
+		bfsize = stmmac_set_bfsize(mtu);
 
 	dma_conf->dma_buf_sz = bfsize;
 	/* Chose the tx/rx size from the already defined one in the
@@ -7773,22 +7774,23 @@ int stmmac_dvr_probe(struct device *device,
 
 	/* MTU range: 46 - hw-specific max */
 	ndev->min_mtu = ETH_ZLEN - ETH_HLEN;
+
 	if (priv->plat->core_type == DWMAC_CORE_XGMAC)
 		ndev->max_mtu = XGMAC_JUMBO_LEN;
-	else if ((priv->plat->enh_desc) || (priv->synopsys_id >= DWMAC_CORE_4_00))
+	else if (priv->plat->enh_desc || priv->synopsys_id >= DWMAC_CORE_4_00)
 		ndev->max_mtu = JUMBO_LEN;
 	else
 		ndev->max_mtu = SKB_MAX_HEAD(NET_SKB_PAD + NET_IP_ALIGN);
-	/* Will not overwrite ndev->max_mtu if plat->maxmtu > ndev->max_mtu
-	 * as well as plat->maxmtu < ndev->min_mtu which is a invalid range.
+
+	/* Warn if the platform's maxmtu is smaller than the minimum MTU,
+	 * otherwise clamp the maximum MTU above to the platform's maxmtu.
 	 */
-	if ((priv->plat->maxmtu < ndev->max_mtu) &&
-	    (priv->plat->maxmtu >= ndev->min_mtu))
-		ndev->max_mtu = priv->plat->maxmtu;
-	else if (priv->plat->maxmtu < ndev->min_mtu)
+	if (priv->plat->maxmtu < ndev->min_mtu)
 		dev_warn(priv->device,
 			 "%s: warning: maxmtu having invalid value (%d)\n",
 			 __func__, priv->plat->maxmtu);
+	else if (priv->plat->maxmtu < ndev->max_mtu)
+		ndev->max_mtu = priv->plat->maxmtu;
 
 	ndev->priv_flags |= IFF_LIVE_ADDR_CHANGE;
 
-- 
2.47.3

Re: [PATCH net-next] net: stmmac: improve ndev->max_mtu setup readability

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Tue, 11 Nov 2025 11:26:34 +0000 you wrote:
> Improve the readibility of the code setting ndev->max_mtu. This depends
> on the hardware specific maximum defined by the MAC core, and also a
> platform provided maximum.
> 
> The code was originally checking that the platform specific maximum was
> between ndev->min_mtu..MAC core maximum before reducing ndev->max_mtu,
> otherwise if the platform specific maximum was less than ndev->min_mtu,
> issuing a warning.
> 
> [...]

Here is the summary with links:
  - [net-next] net: stmmac: improve ndev->max_mtu setup readability
    https://git.kernel.org/netdev/net-next/c/7e975caa0f7b

You are awesome, thank you!
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