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

[Stefan Wahren <stefan.wahren@i2se.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

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
>>