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,

(please don't top-post :-)

"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.

> 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.

> 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.

-- 
balbi

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