[OpenWrt-Devel] 18.06 Bug: Baby Jumbo Frames on mt7621

[Jaap Buurman]

On Thu, May 24, 2018 at 8:00 PM, Martin Blumenstingl
<martin.blumenstingl at googlemail.com> wrote:
> Hello Jaap,
>
> On Thu, May 24, 2018 at 12:00 PM, Jaap Buurman <jaapbuurman at gmail.com> wrote:
>> Dear all,
>>
>> I found some additional information in the system log: Thu May 24
>> 11:38:39 2018 kern.err kernel: [83864.729458] eth0: Invalid MTU 1508
>> requested, hw max 1500
>> Digging deeper, this seems like a message that is spawned by a
>> function in /net/core.dev.c of the linux kernel:
>>
>> if (dev->max_mtu > 0 && new_mtu > dev->max_mtu) {
>> net_err_ratelimited("%s: Invalid MTU %d requested, hw max %d\n",
>> dev->name, new_mtu, dev->max_mtu);
>> return -EINVAL;
>> }
>>
>> Is there anybody that happens to know where exactly this max_mtu value
>> is set to 1500? For mt7621 devices this should be 2048 (Baby jambo
>> frames).
>>
>> Thank you very much in advance.
>>
>> Yours sincerely,
>>
>> Jaap Buurman
>>
>> On Tue, May 22, 2018 at 3:05 PM, Jaap Buurman <jaapbuurman at gmail.com> wrote:
>>> Dear all,
>>>
>>> The switch to the 4.14 kernel apparently broke the baby jumbo frames
>>> support of 2048 bytes that the switch is capable off. I found out that
>>> changing the mtu above 1500 via Luci no longer applies properly.
>>> Trying to manually change the mtu via ssh also fails:
>>>
>>> root at LEDE:~# ifconfig eth0 mtu 1508 up
>>> ifconfig: SIOCSIFMTU: Invalid argument
>>>
>>> If there is any additional information that I can supply, please let
>>> me know. I am also more than willing to help test potential fixes :)
> I *believe* Mathias has a fix for this in his tree (but I'm not sure
> if he has the hardware to test it): [0]
> maybe you can give it a go and report back?
>
>
> Regards
> Martin
>
>
> [0] https://git.openwrt.org/?p=openwrt/staging/mkresin.git;a=commitdiff;h=cc5f1fe7aa02943f3b39ffbd9dc3b8fcad569c8f

Dear Martin and Mathias,

I have compiled and flashed an image from Mathias' tree checked out at
the commit linked in Martin's previous message. Unfortunately, I am
still seeing the following message in dmesg:
[  243.845159] eth0: Invalid MTU 1508 requested, hw max 1500

If there are additional tests you would like me to run, please do not
hesitate to contact me :)

Yours sincerely,

Jaap Buurman

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