Re: [REGRESSION][BISECTED][STABLE] MT7925: mDNS and IPv6 broken in kernel 6.14.3 and above

fossben
2025-04-30T11:38:48Z

Hi Yen,

Here are the reproduction steps.

I have two Arch Linux machines:
(A) one PC (hostname: arch-desktop) connected to the network via MT7925 wifi
(B) one NAS box (hostname: arch-nas) connected to the network via Intel I225-V ethernet

Both machines are running 6.14.4 and have Avahi + nss-mdns enabled according to the Arch Wiki: https://wiki.archlinux.org/title/Avahi#Hostname_resolution

Both machines also have systemd-resolved configured in stub resolver mode but with MulticastDNS=no to avoid conflicting with Avahi. Bug is reproducible without systemd-resolved as well, but I am listing it here for completeness.

Steps:
1. On arch-desktop, attempt to `ping arch-nas.local`.
2. Ping will fail to go through with message: `ping: arch-nas.local: Name or service not known`
3. Downgrade arch-desktop to 6.14.2 or below.
4. `ping arch-nas.local` again.
5. Ping should go through on IPv6 or IPv4. My network has IPv6 enabled so it usually uses an IPv6 local address.

For the issue where the MT7925 wifi is unable to grab an IPv6 address, I believe it occurs when there's no active DHCP lease. Usually happens in the morning when I turn on my PC. If I downgrade to 6.14.2, it will immediately grab a lease. Switching back to 6.14.4 retains that address for a while until I encounter it again next morning.

Thanks for looking into this!
Ben

On Wednesday, April 30th, 2025 at 1:48 AM, Mingyen Hsieh <Mingyen.Hsieh@mediatek.com> wrote:

> On Wed, 2025-04-30 at 01:14 +0000, fossben@pm.me wrote:
> >
> > External email : Please do not click links or open attachments until
> > you have verified the sender or the content.
> >
> >
> > Hello all,
> >
> > After upgrading to 6.14.3 on my PC with a MT7925 chip, I noticed that
> > I could no longer ping *.local addresses provided by Avahi. In
> > addition, I also noticed that I was not able to get a DHCP IPv6
> > address from my router, no matter how many times I rebooted the
> > router or reconnected with NetworkManager.
> >
> > Reverting to 6.14.2 fixes both mDNS and IPv6 addresses immediately.
> > Going back to 6.14.3 immediately breaks mDNS again, but the IPv6
> > address will stay there for a while before disappearing later,
> > possibly because the DHCP lease expired? I am not sure exactly when
> > it stops working.
> >
> > I've done a kernel bisect between 6.14.2 and 6.14.3 and found the
> > offending commit that causes mDNS to fail:
> >
> > commit 80007d3f92fd018d0a052a706400e976b36e3c87
> > Author: Ming Yen Hsieh <mingyen.hsieh@mediatek.com>
> > Date:   Tue Mar 4 16:08:50 2025 -0800
> >
> >     wifi: mt76: mt7925: integrate *mlo_sta_cmd and *sta_cmd
> >
> >     commit cb1353ef34735ec1e5d9efa1fe966f05ff1dc1e1 upstream.
> >
> >     Integrate *mlo_sta_cmd and *sta_cmd for the MLO firmware.
> >
> >     Fixes: 86c051f2c418 ("wifi: mt76: mt7925: enabling MLO when the
> > firmware supports it")
> >
> >  drivers/net/wireless/mediatek/mt76/mt7925/mcu.c | 59 ++++-----------
> > --------------------------------------------
> >  1 file changed, 4 insertions(+), 55 deletions(-)
> >
> > I do not know if this same commit is also causing the IPv6 issues as
> > testing that requires quite a bit of time to reproduce. What I do
> > know with certainty as of this moment is that it definitely breaks in
> > kernel 6.14.3.
> >
> > I've attached my hardware info as well as dmesg logs from the last
> > working kernel from the bisect and 6.14.4 which exhibits the issue.
> > Please let me know if there's any other info you need.
> >
> > Thanks!
> > Benjamin Xiao
>
> Hi,
>
> Thanks for reporting this issue, we will aim into this.
>
> Can you provide me with your testing steps?
>
> Best Regards,
> Yen.
>
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