Re: [PATCH v3] usb: dwc3: gadget: Prevent EP resource conflicts during StartTransfer

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

On Fri, Feb 27, 2026, 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.
> 
> Additionally, a race condition exists between dwc3_gadget_ep_disable()
> and dwc3_gadget_ep_queue() when manipulating dep->flags. When
> dwc3_gadget_ep_disable() calls dwc3_gadget_giveback(), the dwc->lock is
> temporarily released. If dwc3_gadget_ep_queue() runs in that window, it
> may set the DWC3_EP_TRANSFER_STARTED flag as part of
> dwc3_send_gadget_ep_cmd(). When ep_disable resumes, it unconditionally
> clears all flags except those explicitly masked, potentially clearing
> DWC3_EP_TRANSFER_STARTED even though a new transfer has started. This
> leads to "No resource for ep" warnings on subsequent StartTransfer
> attempts.
> 
> The underlying framework issue is that usb_ep_disable() is expected to
> complete pending requests before returning, but is allowed to be called
> from interrupt context where sleeping to wait for completion is not
> possible.
> 
> As temporary workarounds for this framework limitation:
> 
> 1. In __dwc3_gadget_ep_enable(), add a check for the
>    DWC3_EP_TRANSFER_STARTED flag before issuing a new StartTransfer.
>    This prevents a second StartTransfer on an already busy endpoint,
>    eliminating the resource conflict.
> 
> 2. In __dwc3_gadget_ep_disable(), preserve the DWC3_EP_TRANSFER_STARTED
>    flag when masking dep->flags if it is actually set, preventing the
>    race with dwc3_gadget_ep_queue() from corrupting the flag state.
> 
> These changes eliminate the "No resource for ep" warnings and potential
> kernel panics caused by panic_on_warn.
> 
> 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
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Selvarasu Ganesan <selvarasu.g@samsung.com>
> ---
> 
> Note: No Fixes tag is added because this is a workaround for the
> gadget framework issue where the gadget framework calls usb_ep_disable()
> in interrupt context without ensuring endpoint flushing completes.
> A proper fix requires refactoring the framework to make sure
> usb_ep_disable is invoked in process context.
> 
> Changes in v3:
>  - Revised the commit message to detail the real gadget framework issue
>    pointed out by the reviewer.
>  - Merged the two fixes for the same ep wringing into one patch.
> Link to v2: https://urldefense.com/v3/__https://lore.kernel.org/linux-usb/20251117155920.643-1-selvarasu.g@samsung.com/__;!!A4F2R9G_pg!cQzQQ5kAWF6CE5hQe7VqFdnaxqwzsTB1ZGNT1GvCH28GoB_nESZR5Y2jtxdZBls6wBIM4OtpvG4dSaylvNC3qbh547k$ 
> 
> 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!cQzQQ5kAWF6CE5hQe7VqFdnaxqwzsTB1ZGNT1GvCH28GoB_nESZR5Y2jtxdZBls6wBIM4OtpvG4dSaylvNC38z-CRD4$ 
> ---
>  drivers/usb/dwc3/gadget.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> index 0a688904ce8c5..3af1bbfe3d92b 100644
> --- a/drivers/usb/dwc3/gadget.c
> +++ b/drivers/usb/dwc3/gadget.c
> @@ -971,8 +971,9 @@ 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;
> @@ -1096,6 +1097,23 @@ static int __dwc3_gadget_ep_disable(struct dwc3_ep *dep)
>  	 */
>  	if (dep->flags & DWC3_EP_DELAY_STOP)
>  		mask |= (DWC3_EP_DELAY_STOP | DWC3_EP_TRANSFER_STARTED);
> +
> +	/*
> +	 * When dwc3_gadget_ep_disable() calls dwc3_gadget_giveback(),
> +	 * the dwc->lock is temporarily released. If dwc3_gadget_ep_queue()
> +	 * runs in that window it may set the DWC3_EP_TRANSFER_STARTED flag as
> +	 * part of dwc3_send_gadget_ep_cmd. The original code cleared the flag
> +	 * unconditionally in the mask operation, which could overwrite the
> +	 * concurrent modification.
> +	 *
> +	 * As a workaround for the interrupt context constraint where we cannot
> +	 * wait for endpoint flushing, preserve the DWC3_EP_TRANSFER_STARTED
> +	 * flag if it is set, avoiding resource conflicts until the framework
> +	 * is fixed to properly synchronize endpoint lifecycle management.
> +	 */
> +	if (dep->flags & DWC3_EP_TRANSFER_STARTED)
> +		mask |= DWC3_EP_TRANSFER_STARTED;
> +
>  	dep->flags &= mask;
>  
>  	/* Clear out the ep descriptors for non-ep0 */
> -- 
> 2.34.1
> 

Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>

Thanks!
Thinh