Re: [PATCH v3] usb: dwc3: core: Fix RAM interface getting stuck during enumeration

[Krishna Kurapati]

On 9/29/2026 10:49 AM, Krishna Kurapati wrote:
> During plug-in/plug-out test cases, it is sometimes seen that no events
> are generated by the controller and all CSR register reads give "0" and
> CSR_Timeout bit gets set indicating that CSR reads/writes are timing out
> or timed out.
>
> The issue comes up on different instnaces of enumeration on different
> platforms. On SM8550, the debug log is as follows:
>
> Prepared a TRB on ep0out and did start transfer to get set
> address request from host:
>
> <...>-7191    [000] D..1.    66.421006: dwc3_gadget_ep_cmd: ep0out:
> cmd 'Start Transfer' [406] params 00000000 efffa000 00000000 -->
> status: Successful
>
> <...>-7191    [000] D..1.    66.421196: dwc3_event: event (0000c040):
> ep0out: Transfer Complete (sIL) [Setup Phase]
>
> <...>-7191    [000] D..1.    66.421197: dwc3_ctrl_req: Set
> Address(Addr = 01)
>
> An XFER NRDY is received on ep0in for zero length status phase and
> a Start Transfer was done on ep0in with 0-length packet in 2 Stage
> status phase:
>
> <...>-7191    [000] D..1.    66.421249: dwc3_event: event (000020c2):
> ep0in: Transfer Not Ready [00000000] (Not Active) [Status Phase]
>
> <...>-7191    [000] D..1.    66.421266: dwc3_prepare_trb: ep0in: trb
> ffffffc00fcfd000 (E0:D0) buf 00000000efffa000 size 0 ctrl 00000c33
> sofn 00000000 (HLcs:SC:status2)
>
> <...>-7191    [000] D..1.    66.421387: dwc3_gadget_ep_cmd: ep0in: cmd
> 'Start Transfer' [406] params 00000000 efffa000 00000000 -->status:
> Successful
>
> A bus reset was then received directly after 500 msec. Software never
> got the cmd complete for the start transfer done in status phase. Here
> the RAM interface is stuck. So host issues a bus reset as link is
> idle for 500 msec:
>
> <...>-7191    [000] D..1.    66.935603: dwc3_event: event (00000101):
> Reset [U0]
>
> Then software sees that it is in status phase and we issue an ENDXFER
> on ep0in and it gets timedout waiting for the CMDACT to go '0':
>
> <...>-7191    [000] D..1.    66.958249: dwc3_gadget_ep_cmd: ep0in: cmd
> 'End Transfer' [10508] params 00000000 00000000 00000000 --> status:
> Timed Out
>
> Upon debug with Synopsys, the root cause is as follows:
>
> During any transfer, if the data is not successfully transmitted,
> then a Done (with failure) handshake is returned, so that the BMU
> can re-attempt the same data again by rewinding its data pointers.
>
> But, if the USB IN is a 0-length payload (which is what is happening
> in this case - 2 stage status phase of set_address), then there is no
> need to rewind the pointers and the Done (with failure) handshake is
> not returned for failure case. This keeps the Request-Done interface
> busy till the next Done handshake. The MAC sends the 0-length payload
> again when the host requests. If the transmission is successful this
> time, the Done (with success) handshake is provided back. Otherwise,
> it repeats the same steps again.
>
> If the cable is disconnected or if the Host aborts the transfer on 3
> consecutive failed attempts, the Request-Done handshake is not
> complete. This keeps the interface busy.
>
> The subsequent RAM access cannot proceed until the above pending
> transfer is complete. This results in failure of any access to RAM
> address locations. Many of the EndPoint commands need to access the
> RAM and they would fail to complete successfully.
>
> Furthermore when cable removal happens, this would not generate a
> disconnect event and the "connected" flag remains true always blockin
> suspend.
>
> Synopsys confirmed that the issue is present on all USB3 devices and
> as a workaround, suggested to re-initialize device mode.
>
> Signed-off-by: Krishna Kurapati <krishna.kurapati@xxxxxxxxxxxxxxxx>


Thinh, Can you help provide your review on this patch.

Regards,
Krishna,