Date:   Mon, 11 Feb 2019 12:06:36 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
To:     Stefan Wahren <stefan.wahren@i2se.com>
Cc:     Stanislaw Gruszka <sgruszka@redhat.com>,
        Felix Fietkau <nbd@nbd.name>,
        Doug Anderson <dianders@chromium.org>,
        Minas Harutyunyan <hminas@synopsys.com>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        linux-usb@vger.kernel.org
Subject: Re: [BUG] mt76x0u: Probing issues on Raspberry Pi 3 B+
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> Hi,
> 
> Am 11.02.19 um 11:04 schrieb Lorenzo Bianconi:
> >> On Sun, Feb 10, 2019 at 11:22:25AM +0100, Lorenzo Bianconi wrote:
> >>>> On Sat, Feb 09, 2019 at 09:29:05PM +0100, Stefan Wahren wrote:
> >>>>>> could you please test the following series:
> >>>>>> https://patchwork.kernel.org/cover/10764453/
> >>>>> yeah this fixed the probing timeout and the driver will probe successful. AFAIK the dwc2 host mode doesn't support scatter-gather yet.
> >>>> So this is either dwc2 scatter-gather problem which should be addressed in
> >>>> this driver or mt76x0u does something wrong when configuring SG.
> >>>>
> >>>> Disabling SG is just workaround, which do not address actual problem.
> >>>>
> >>>> I think I found mt76x0u issue that could cause this USB probe error
> >>>> (and possibly also address AMD IOMMU issue). We seems do not correctly
> >>>> set URB transfer length smaller than sg buffer length. Attached  patch
> >>>> should correct that.
> >>> Hi Stanislaw,
> >>>
> >>> I think 'sg[0].length' is already set in mt76u_fill_rx_sg().
> >> It is, buf->len and sg[0].length are initialized to the same value for 1
> >> segment. But then buf->len (assigned to urb->buffer_transfer_length) change
> >> to smaller value , but sg[0].length stay the same. What I think can be
> >> problem for usb host driver.
> >>
> >>> Moreover applying this patch I got the following crash (rpi-5.0.y):
> >> Ok, so with patch probe fail instantly and trigger yet another bug(s)
> >> on error path. You seems to address that already. 
> >>
> >>> Moreover for mt76x0u SG is 'already' disabled since we use just one
> >>> buffer so from performance point of view I do not see any difference
> >>> of using a standard usb buffer.
> >>> This patch has been tested in multiple scenarios and seems to fix
> >>> reported issues (for usb2.0).
> >> Ok, so passing buffer via urb->transfer_buffer works. But why urb->sg
> >> does not work for 1 segment ?
> > Here it is a different issue respect to the AMD IOMMU one, dwc2 host driver
> > does not implement SG I/O so probing fails. I guess it is still useful to
> > implement a 'legacy' mode that enable mt76 on host controllers that do not implement
> > SG I/O (rpi is a very common device so it will be cool to have mt76 working on
> > it). Moreover we are not removing functionalities, user experience will remain
> > the same
> >
> i'm not sure that you understand my mail [1] with the summary of my test
> results.
> 

Yes right, I did not get it sorry :)
as indicated here https://www.raspberrypi.org/documentation/linux/kernel/building.md
I am using bcm2709_defconfig config (using it I spotted the mt76 crashes and
probe failure)

Regards,
Lorenzo

> In case of using the arm/multi_v7_defconfig (32 bit) the mt76 works like
> a charm without your sg avoid patch series, but the arm64/defconfig (64
> bit) requires the series to probe at least. So i wouldn't conclude from
> the fact that dwc2 doesn't support SG any probing issues on arm64. So we
> need to investigate which config option triggers the problem.
> 
> Stefan
> 
> [1] - https://marc.info/?l=linux-usb&m=154981675724078
>