Re: Issue with DWC3 Gadget Driver: Stall After Transmitting Only 2KB Using Scatter-Gather and TRB Chaining

[Thinh Nguyen <Thinh.Nguyen@synopsys.com>]

On Mon, Nov 03, 2025, Li Haifeng wrote:
> Dear Thinh,
> 
> Thank you for your detailed analysis and suggestions.
> 
> Using a logic analyzer, I did not observe any NRDY or flow control
> events on the device side for the corresponding endpoint. I reviewed
> the handling logic in the USB ECM host module from the latest Linux
> kernel. It allocates the receive buffer based on wMaxSegmentSize,
> which is 1514 bytes, so if the data sent from the device exceeds this
> value (as the previous 2KB size did), the host's behavior might become
> abnormal. I will conduct an experiment to observe this phenomenon.
> 
> What puzzles me is why the device's endpoint does not generate an NRDY
> event in this scenario.
> 

The host drives the device. The device would respond with NRDY if the
host requests for data. If not, then probably host didn't request for
it. You should be able to confirm that in the USB traffic trace. This is
correspond to what you noted that host prepared and likely requested
1514 bytes only. There's a mismatch in the communication at the gadget
driver protocol. I'm not familiar with the Ethernet protocol, check
that. The device should not send more than what the host asks for.

BR,
Thinh