[BUG] mt76x0u: Probing issues on Raspberry Pi 3 B+

[Lorenzo Bianconi <lorenzo.bianconi@redhat.com>]

> Hi Lorenzo,
> 
> Am 11.02.19 um 12:06 schrieb Lorenzo Bianconi:
> >> 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)
> 
> no problem, at the beginning this could be very confusing. I only want
> to clarify that this documentation refers to the vendor kernel (with a
> different USB host driver) of the Raspberry Pi Foundation.
> 
> All my results refers to the mainline kernel we all should talk about. I
> started a gist which try to describe the mainline variant:
> https://gist.github.com/lategoodbye/c7317a42bf7f9c07f5a91baed8c68f75

So to summarize:
- Raspberry Pi Foundation kernel works just with RFC series
- mainline kernel works out of the box

is my understanding correct? I am still considering adding a legacy mode since
there will not be any regression using it instead of SG I/O with just one SG
buffer

Regards,
Lorenzo

> 
> Maybe this could be helpful.
> 
> Stefan
> 
> > 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
> >>