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Question asked by Naishab KP on Nov 12, 2018
Latest reply on Nov 25, 2018 by Pavel Chubakov


My question is regarding JUMBO frame enabling on T4241 processor.

I am using linux-4.17.3 from fsl git. I have used fsl_fm_max_frm=9600 parameter in U-Boot bootargs to set preferred MTU.

But when i set MTU to any other value than 1500 using ifconfig after booting to linux, it gives error saying  'hw max' is 1522 only. 

In T4240RDB this (fsl_fsl_max_frm) used to work (i think SDK-1.7 Linux kernel 3.10).

When i digged more into the fsl ethernet driver src code i noticed that .no_change_mtu function is removed.


Then i changed the file 'linux-fslc-4.17.3/drivers/net/ethernet/freescale/fman/fman.c' to


Now even if i set MTU to any value (say 6000), Receiving is  working for up to 9600 and transmit frames are obeying the set MTU (6000).


Looks like bootargs parameter (fsl_fm_max_frm) is not working properly (which used to work in Linux-3.x).

See the following lines from freescale/fman/fman.c

/* Max frame size, across all interfaces. Configurable from bootargs, to avoid allocating oversized (socket)
 * buffers when not using jumbo frames. Must be large enough to accommodate the network MTU, but small enough
 * to avoid wasting skb memory. Could be overridden once, at boot-time, via the fm_set_max_frm() callback.

static int fsl_fm_max_frm = FSL_FM_MAX_FRAME_SIZE;
module_param(fsl_fm_max_frm, int, 0);
MODULE_PARM_DESC(fsl_fm_max_frm, "Maximum frame size, across all interfaces");

Also from  'linux-fslc-4.17.3/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c'
static int dpaa_eth_probe(struct platform_device *pdev)

       /* If fsl_fm_max_frm is set to a higher value than the all-common 1500, we choose conservatively and let the user explicitly set a higher
         * MTU via ifconfig. Otherwise, the user may end up with different MTUs in the same LAN.

          * If on the other hand fsl_fm_max_frm has been chosen below 1500,  start with the maximum allowed.        

        net_dev->mtu = min(dpaa_get_max_mtu(), ETH_DATA_LEN);

It looks like frame size fixed compile time (either 1500 or 9600).

Could not find the function  fm_set_max_frm () anywhere in driver src code!!!

Why 9k Frame reception is working even if MTU is set to 6k only,  but Tx side it is working in configured MTU size.

Can anyone help?