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

Jaap Buurman jaapbuurman at gmail.com
Fri May 25 05:48:06 EDT 2018

On Fri, May 25, 2018 at 11:30 AM, Jaap Buurman <jaapbuurman at gmail.com> wrote:
> 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

Dear Martin, Mathias and the rest,

Please scratch my previous message. It seems like the flash was not
successful, and hence I was still running the old firmware. However, I
have tried flashing 3 different times now, without any luck. The
router ends up rebooting and boots right into the old firmware. This
seems to be a major bug. Is there anything I can do to help debug this
particular issue? Seems like a dealbreaker for 18.06 (which I am
running now) to me. I could simply use recovery and flash a firmware
like that, but I would prefer to get to the bottom of this issue so
that end users won't end up stuck on a particular firmware. Any ideas
what I could do to debug this?

Yours sincerely,

Jaap Buurman

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