Re: [PATCH v2] usb: dwc3: gadget: Prevent EPs resource conflict during StartTransfer

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

On Mon, Nov 17, 2025, Selvarasu Ganesan wrote:
> The below “No resource for ep” warning appears when a StartTransfer
> command is issued for bulk or interrupt endpoints in
> `dwc3_gadget_ep_enable` while a previous StartTransfer on the same
> endpoint is still in progress. The gadget functions drivers can invoke
> `usb_ep_enable` (which triggers a new StartTransfer command) before the
> earlier transfer has completed. Because the previous StartTransfer is
> still active, `dwc3_gadget_ep_disable` can skip the required
> `EndTransfer` due to `DWC3_EP_DELAY_STOP`, leading to  the endpoint
> resources are busy for previous StartTransfer and warning ("No resource
> for ep") from dwc3 driver.
> 
> To resolve this, a check is added to `dwc3_gadget_ep_enable` that
> checks the `DWC3_EP_TRANSFER_STARTED` flag before issuing a new
> StartTransfer. By preventing a second StartTransfer on an already busy
> endpoint, the resource conflict is eliminated, the warning disappears,
> and potential kernel panics caused by `panic_on_warn` are avoided.
> 
> ------------[ cut here ]------------
> dwc3 13200000.dwc3: No resource for ep1out
> WARNING: CPU: 0 PID: 700 at drivers/usb/dwc3/gadget.c:398 dwc3_send_gadget_ep_cmd+0x2f8/0x76c
> Call trace:
>  dwc3_send_gadget_ep_cmd+0x2f8/0x76c
>  __dwc3_gadget_ep_enable+0x490/0x7c0
>  dwc3_gadget_ep_enable+0x6c/0xe4
>  usb_ep_enable+0x5c/0x15c
>  mp_eth_stop+0xd4/0x11c
>  __dev_close_many+0x160/0x1c8
>  __dev_change_flags+0xfc/0x220
>  dev_change_flags+0x24/0x70
>  devinet_ioctl+0x434/0x524
>  inet_ioctl+0xa8/0x224
>  sock_do_ioctl+0x74/0x128
>  sock_ioctl+0x3bc/0x468
>  __arm64_sys_ioctl+0xa8/0xe4
>  invoke_syscall+0x58/0x10c
>  el0_svc_common+0xa8/0xdc
>  do_el0_svc+0x1c/0x28
>  el0_svc+0x38/0x88
>  el0t_64_sync_handler+0x70/0xbc
>  el0t_64_sync+0x1a8/0x1ac
> 
> Fixes: a97ea994605e ("usb: dwc3: gadget: offset Start Transfer latency for bulk EPs")
> Cc: stable@vger.kernel.org
> Signed-off-by: Selvarasu Ganesan <selvarasu.g@samsung.com>
> ---
> 
> Changes in v2:
> - Removed change-id.
> - Updated commit message.
> Link to v1: https://urldefense.com/v3/__https://lore.kernel.org/linux-usb/20251117152812.622-1-selvarasu.g@samsung.com/__;!!A4F2R9G_pg!dQNEaMvjzrOiP0c9U6lyj31ysXGkjBAs8c_KjgDCp6ONSZcTrF15DXaJeFTK02v0RLS3w0NQ2K5_pvXak_7c8tIxKL8$ 
> ---
>  drivers/usb/dwc3/gadget.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> index 1f67fb6aead5..8d3caa71ea12 100644
> --- a/drivers/usb/dwc3/gadget.c
> +++ b/drivers/usb/dwc3/gadget.c
> @@ -963,8 +963,9 @@ static int __dwc3_gadget_ep_enable(struct dwc3_ep *dep, unsigned int action)
>  	 * Issue StartTransfer here with no-op TRB so we can always rely on No
>  	 * Response Update Transfer command.
>  	 */
> -	if (usb_endpoint_xfer_bulk(desc) ||
> -			usb_endpoint_xfer_int(desc)) {
> +	if ((usb_endpoint_xfer_bulk(desc) ||
> +			usb_endpoint_xfer_int(desc)) &&
> +			!(dep->flags & DWC3_EP_TRANSFER_STARTED)) {
>  		struct dwc3_gadget_ep_cmd_params params;
>  		struct dwc3_trb	*trb;
>  		dma_addr_t trb_dma;
> -- 
> 2.34.1
> 

Thanks for the catch. The problem is that the "ep_disable" process
should be completed after usb_ep_disable is completed. But currently it
may be an async call.

This brings up some conflicting wording of the gadget API regarding
usb_ep_disable. Here's the doc regarding usb_ep_disable:

	/**
	 * usb_ep_disable - endpoint is no longer usable
	 * @ep:the endpoint being unconfigured.  may not be the endpoint named "ep0".
	 *
	 * no other task may be using this endpoint when this is called.
	 * any pending and uncompleted requests will complete with status
	 * indicating disconnect (-ESHUTDOWN) before this call returns.
	 * gadget drivers must call usb_ep_enable() again before queueing
	 * requests to the endpoint.
	 *
	 * This routine may be called in an atomic (interrupt) context.
	 *
	 * returns zero, or a negative error code.
	 */

It expects all requests to be completed and given back on completion. It
also notes that this can also be called in interrupt context. Currently,
there's a scenario where dwc3 may not want to give back the requests
right away (ie. DWC3_EP_DELAY_STOP). To fix that in dwc3, it would need
to "wait" for the right condition. But waiting does not make sense in
interrupt context. (We could busy-poll to satisfy the interrupt context,
but that doesn't sound right either)

This was updated from process context only to may be called in interrupt
context:

b0d5d2a71641 ("usb: gadget: udc: core: Revise comments for USB ep enable/disable")


Hi Alan,

Can you help give your opinion on this?

Thanks,
Thinh