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

[Nicolai Buchwitz <nb@tipi-net.de>]

On 10.9.2026 17:14, Maxime Chevallier wrote:
> 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

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>

Thanks,
Nicolai