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

[Stefan Wahren <stefan.wahren@i2se.com>]

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.

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