LWIP buffer alignment

[Jerome Forissier]

On 9/8/25 18:44, Tim Harvey wrote:
> On Mon, Sep 8, 2025 at 7:21 AM Jerome Forissier
> <jerome.forissier at linaro.org> wrote:
>>
>> Hi Tim,
>>
>> On 9/5/25 19:45, Tim Harvey wrote:
>>> Hi Jerome,
>>>
>>> I'm working with a board that has an IMX ENETC MAC on it
>>> drivers/net/fsl_enetc.c and when configured for LWIP it fails and
>>> warns about RX buffers not being 64B aligned:
>>> WARNING at drivers/net/fsl_enetc.c:776/enetc_setup_rx_bdr()!
>>>
>>> I tried changing MEM_ALIGNMENT to 64 in lib/lwip/u-boot/lwipopts.h but
>>> that did not align the buffers to 64B. Where are the lwip buffers
>>> created and do you have any suggestions for working around this?
>>
>> The LwIP RX buffers are created in net-lwip.c. See eth_init_rings().
>> You might have PKTSIZE_ALIGN and/or PKTALIGN set wrongly.
>>
> 
> Hi Jerome,
> 
> Thanks for pointing me to eth_init_rings(). This function uses
> net_pkt_buf as the base for the net_rx_packets array which is a static
> global and not aligned.
> 
> Looks like we need:
> diff --git a/net/lwip/net-lwip.c b/net/lwip/net-lwip.c
> index 660ceb10cbe..aec05d55de6 100644
> --- a/net/lwip/net-lwip.c
> +++ b/net/lwip/net-lwip.c
> @@ -30,7 +30,7 @@ void (*push_packet)(void *, int len) = 0;
>  static int net_try_count;
>  static int net_restarted;
>  int net_restart_wrap;
> -static uchar net_pkt_buf[(PKTBUFSRX) * PKTSIZE_ALIGN + PKTALIGN];
> +static uchar net_pkt_buf[(PKTBUFSRX) * PKTSIZE_ALIGN + PKTALIGN]
> __attribute__((aligned(PKTALIGN)));
>  uchar *net_rx_packets[PKTBUFSRX];
>  uchar *net_rx_packet;
>  const u8 net_bcast_ethaddr[6] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff };
> 
> If you agree I'll submit that. This does resolve the issue I see with
> the enetc driver using LwIP.

Please do. But then I wonder how come the legacy NET works OK? Or does it?
net/net.c has the same initialization sequence.

Regards,
-- 
Jerome

> 
> Best Regards,
> 
> Tim