Re: [PATCH net v4 5/7] net: stmmac: dwmac4: Use the correct bufzise when the len is exactly 8K

[Nicolai Buchwitz]

Hi Maxime

On 17.9.2026 23:53, Maxime Chevallier wrote:
> DMA bufsize selection isn't made on the MTU but the actual frame 
> length,
> so including the L2 header. On DWMAC4, if the len is exactly 
> BUF_SIZE_8KiB,
> the next larger size is incorrectly selected.
>
> Lets fix the comparison and while at it, rename the parameter from len
> to mtu.

Isn't it "[...] from mtu to len"?

> Fixes: c3efed5ad1b0 ("net: stmmac: Enable dwmac4 jumbo frame more than 
> 8KiB").
> Signed-off-by: Maxime Chevallier <maxime.chevallier@xxxxxxxxxxx>
> ---
>  drivers/net/ethernet/stmicro/stmmac/dwmac4_descs.c | 4 ++--
>  drivers/net/ethernet/stmicro/stmmac/hwif.h         | 2 +-
>  drivers/net/ethernet/stmicro/stmmac/ring_mode.c    | 4 ++--
>  3 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac4_descs.c 
> b/drivers/net/ethernet/stmicro/stmmac/dwmac4_descs.c
> index 2994df41ec2c..c6a8f8d73501 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/dwmac4_descs.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/dwmac4_descs.c
> @@ -474,11 +474,11 @@ static void dwmac4_set_sarc(struct dma_desc *p, 
> u32 sarc_type)
>  					  sarc_type));
>  }
>
> -static int set_16kib_bfsize(int mtu)
> +static int set_16kib_bfsize(int len)
>  {
>  	int ret = 0;
>
> -	if (unlikely(mtu >= BUF_SIZE_8KiB))
> +	if (unlikely(len > BUF_SIZE_8KiB))

Should we drop set_16kib_bfsize() instead?

After 6/7 it looks to me like the same test as the first branch of
stmmac_set_bfsize(). The caller ends up there anyway when it returns 0
and chain_mode_ops doesn't have it at all.

>  		ret = BUF_SIZE_16KiB;
>  	return ret;
>  }
> diff --git a/drivers/net/ethernet/stmicro/stmmac/hwif.h 
> b/drivers/net/ethernet/stmicro/stmmac/hwif.h
> index 04dafec021b4..de7aa6d28148 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/hwif.h
> +++ b/drivers/net/ethernet/stmicro/stmmac/hwif.h
> @@ -540,7 +540,7 @@ struct stmmac_mode_ops {
>  	bool (*is_jumbo_frm)(unsigned int len, bool enh_desc);
>  	int (*jumbo_frm)(struct stmmac_tx_queue *tx_q, struct sk_buff *skb,
>  			 int csum);
> -	int (*set_16kib_bfsize)(int mtu);
> +	int (*set_16kib_bfsize)(int len);
>  	void (*init_desc3)(struct dma_desc *p);
>  	void (*refill_desc3)(struct stmmac_rx_queue *rx_q, struct dma_desc 
> *p);
>  	void (*clean_desc3)(struct stmmac_tx_queue *tx_q, struct dma_desc 
> *p);
> diff --git a/drivers/net/ethernet/stmicro/stmmac/ring_mode.c 
> b/drivers/net/ethernet/stmicro/stmmac/ring_mode.c
> index f7949419eb9f..d2f0c321661d 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/ring_mode.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/ring_mode.c
> @@ -124,10 +124,10 @@ static void clean_desc3(struct stmmac_tx_queue 
> *tx_q, struct dma_desc *p)
>  		p->des3 = 0;
>  }
>
> -static int set_16kib_bfsize(int mtu)
> +static int set_16kib_bfsize(int len)
>  {
>  	int ret = 0;
> -	if (unlikely(mtu > BUF_SIZE_8KiB))
> +	if (unlikely(len > BUF_SIZE_8KiB))
>  		ret = BUF_SIZE_16KiB;
>  	return ret;
>  }

regards
Nicolai