RE: [BUG REPORT] usb: dwc3: Bug while setting the USB transfer bandwidth on UVC gadget driver

[Felipe Balbi <balbi@kernel.org>]

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Hi,

"Chow, Watson" <Watson.Chow@Avnet.com> writes:
> Hi,
>
>>Hi,
>>
>>(please don't top-post :-)
> I have tried my best to meet the format requirement

Thanks

>>"Chow, Watson" <Watson.Chow@Avnet.com> writes:
>>> Balbi,
>>>
>>> Thanks for your quick reply.
>>>
>>> Some questions
>>>
>>> 1. You mentioned that the max bandwidth in isoc mode (USB3.0) should be 
>>> around 4Gbps.  
>>>
>>> I have the below calcuation on bandwidth:
>>> In USB3.0, 1 micro frame would take 125us and can transfer max 45000 bytes
>>> So, in 1 sec, we will have 8000 micro frames
>>>
>>> Max bandwidth = 8000 x 4500 x 8 = 2.88Gbps
>>>
>>> Is my understanding correct?
>>
>>probably, It's been a while since I've dug through the spec, to be frank
>>
>>> 2. To achieve the max throughput, I need to configure the uvc gadget driver 
>>> with below parameters. Am I right?
>>>
>>> # modprobe g_webcam streaming_maxpacket=3072 streaming_maxburst=15 
>>> streaming_interval=1
>>
>>right, but there's an assumption here that the gadget will be able to
>>feed data in a timely manner.
>
> How does the DWC3 driver or the gadget driver handle the case with intermittent
> drop of the input video streaming?
>
> Any recover mechanism?

yeah, the missed ISOC is reported to the gadget driver and that has to
queue new requests.

>>> 3. You suggest me to try on kernel v5.12 or the latest v5.13-rc. It looks not
>>> easy in my side to upgrade the kernel version. It would affect those other 
>>> device drivers I'm currently using. So, do you think there's any short cut 
>>> to fix this problem under my current kernel version - v5.4?
>>
>>In that case, you need to ask for support from whoever forces you to
>>stay with such an old kernel. I believe that would be Xilinx.
>
> I have a thought to back port those changes around the dwc3 and gadget driver
> from the latest kernel version to my kernel (v5.4). Do you think this is 
> feasible?

should be, but it's likely quite a bit of work:

$ git rev-list --count v5.4..linus/master -- drivers/usb/dwc3/
257

>>> 4. I read through the procedures to capture debug info by debugfs. However,
>>> in my test with "streaming_maxburst" set to 10 or above, my system would 
>>> crash and I can't pick the log from that point. Any suggestion?
>>
>>have a look at ftrace_dump_on_oops.
>
> I will explore how to enable this
>
>>
>
> Btw, do you know which SoC platform can run the UVC gadget in max throughput.
> Raspberry Pi/TI Beaglebone/i.MX ???

Raspberry Pi uses dwc2
Beaglebone uses musb
i.MX, I think some of them use dwc3 at least.

-- 
balbi

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