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From:   Rosen Penev <rosenp@gmail.com>
Date:   Wed, 16 Jan 2019 09:40:34 -0800
Message-ID: <CAKxU2N_RZ56DFD2TvmyV_4q628fMuQmy9+0Bptw0zoG5U=4egw@mail.gmail.com>
Subject: Re: [RFC 0/4] do not use sg if not properly supported by usb controller
To:     Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Cc:     Stanislaw Gruszka <sgruszka@redhat.com>,
        Felix Fietkau <nbd@nbd.name>,
        linux-wireless <linux-wireless@vger.kernel.org>
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On Tue, Jan 15, 2019 at 7:47 AM Lorenzo Bianconi
<lorenzo.bianconi@redhat.com> wrote:
>
> > On Tue, Jan 15, 2019 at 01:33:41PM +0100, Lorenzo Bianconi wrote:
> > > Use linear fragment and not a single usb scatter-gather buffer in mt76u
> > > {tx,rx} datapath if the usb controller has sg data length constraints.
> > > Moreover add disable_usb_sg module parameter in order to explicitly
> > > disable scatter-gather. Some users have reported sg issues on AMD IOMMU
>
> Hi Stanislaw,
>
> >
> > Not sure what is the problem , but this patch set look like a workaround
> > not fix. If this an issue with IOMMU and sg, seems there is something wrong
> > in sg page mappings eigher on mt76 dirver or IOMMU driver.
>
> The main point here I guess is we do not need sg if fragment number is one (e.g
> usb2.0). Moreover this can fix IOMMU reported issues.
>
> @Rosen: could you please try this series enabling IOMMU?
I will try it later today.

Note that the same behavior occurs with USB 2.0 ports as well. I do
feel that the actual bug is in the IOMMU driver.
>
> >
> > If things need to be fixed in mt76 I whould check if page mappings for
> > sg are correct. Or remove sg usage from mt76_usb completly, mt76 MMIO
> > version do not use sg for framgments, so most likely USB don't need it
> > as well.
>
> usb scatter-gather is used to properly support non-linear skbs (A-MSDU,
> with usb3.0) since the hw (unlike pci counterpart) does not support it,
> so we need it.
>
> Regards,
> Lorenzo
>
> >
> > Thanks
> > Stanislaw