Date:   Tue, 15 Jan 2019 10:04:01 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
To:     Rosen Penev <rosenp@gmail.com>
Cc:     linux-wireless <linux-wireless@vger.kernel.org>
Subject: Re: MT76x2U crashes XHCI driver on AMD Ryzen system
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> On Mon, Jan 14, 2019 at 1:18 AM Lorenzo Bianconi
> <lorenzo.bianconi@redhat.com> wrote:
> >
> > > On Sun, Jan 13, 2019 at 11:00 AM Lorenzo Bianconi
> > > <lorenzo.bianconi@redhat.com> wrote:
> > > >
> > > > >
> > > > >
> > > > > On Sun, Jan 13, 2019 at 5:33 AM, Lorenzo Bianconi <lorenzo.bianconi@redhat.com> wrote:
> > > > >
> > > > > Direct. No VM used. This is the only peripheral causing this issue.
> > > > >
> > > > > Is the device connected to a usb3.0 port? If so, could you please try to connect the dongle to a 2.0 one?
> > > > >
> > > > > I tried through a USB 2.0 port. Shouldn't make a difference as they both use the xhci driver.
> > > > >
> > > >
> > > > mt76x2u supports scatter-gather on usb 3.0 (not on 2.0)
> > > Tried a USB 3 port. Same result.
> > > >
> > > > > Could you please double check if IOMMU is enabled?
> > > > >
> > > >
> > > > Have you tried to disable it? Does it make any difference?
> > > No idea how. UEFI doesn't seem to show anything similar.
> > >
> > > Similar bug report: https://bugzilla.kernel.org/show_bug.cgi?id=202241
> >
> > You should be able to disable iommu using GRUB_CMDLINE_LINUX in
> > /etc/default/grub (I guess setting iommu=off and reinstalling grub)
> > https://wiki.gentoo.org/wiki/IOMMU_SWIOTLB
> Yep. Working great now. I wonder what mt76 is doing to cause the crash though...

Thanks for bisecting the issue. I think amd iommu does not support well usb scatter-gather
(used by default in mt76u). I am working on a series in order to add the possibility to
disable it.

Regards,
Lorenzo

> >
> > Regards,
> > Lorenzo