Re: [PATCH] usb: dwc3: Skip TRBs while removing requests in disconnect path

From: Elson Serrao
Date: Wed May 31 2023 - 20:57:47 EST

On 5/31/2023 4:20 PM, Thinh Nguyen wrote:
On Wed, May 31, 2023, Elson Roy Serrao wrote:
Consider a scenario where cable disconnect happens when there is an active
usb reqest queued to the UDC. As part of the disconnect we would issue an
end transfer with no interrupt-on-completion before giving back this
request. Since we are giving back the request without skipping TRBs the
num_trbs field of dwc3_request still holds the stale value previously used.
Function drivers re-use same request for a given bind-unbind session and
hence their dwc3_request context gets preserved across cable
disconnect/connect. When such a request gets re-queued after cable connect,

Why would we preserve the request after a disconnect? The request is
associated with an endpoint, and after disconnect, the endpoint is no
longer valid. Shouldn't the request be freed then?

Function drivers generally allocate usb requests during bind when an endpoint is allocated to it (through usb_ep_autoconfig). These requests are freed when an unbind is called as the function is no longer associated with any end point. The function driver is free to re-use these requests throughout this bind-unbind session. The only restriction is that the function drivers wont be able to queue any requests as long as the endpoint is disabled. But that doesn't enforce function drivers to free the requests with ep_disable(). Even though the endpoint is disabled with cable disconnect, that endpoint is still associated with that particular function driver until that function is unbound.

As an example below is how f_ncm driver allocates and frees the requests during bind/unbind

ep = usb_ep_autoconfig(cdev->gadget, &fs_ncm_notify_desc);
if (!ep)
goto fail;
ncm->notify = ep;

status = -ENOMEM;

/* allocate notification request and buffer */
ncm->notify_req = usb_ep_alloc_request(ep, GFP_KERNEL);

The endpoint is enabled later when set_alt is received and disabled in ncm_disable when the connection goes down (cable disconnect scenario)

usb_ep_free_request(ncm->notify, ncm->notify_req);

I see similar implementation in other function drivers as well. That is, keep the usb requests allocated throughout the bind-unbind session and independent of ep_enable/ep_disable .


we would increase the num_trbs field on top of the previous stale value
thus incorrectly representing the number of TRBs used. Fix this by invoking
skip_trbs() in the ep disable path.

Signed-off-by: Elson Roy Serrao <quic_eserrao@xxxxxxxxxxx>
drivers/usb/dwc3/gadget.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
index 578804d..b45e917 100644
--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c
@@ -986,6 +986,8 @@ static int __dwc3_gadget_ep_enable(struct dwc3_ep *dep, unsigned int action)
return 0;
+static void dwc3_gadget_ep_skip_trbs(struct dwc3_ep *dep, struct dwc3_request *req);
void dwc3_remove_requests(struct dwc3 *dwc, struct dwc3_ep *dep, int status)
struct dwc3_request *req;
@@ -1000,6 +1002,7 @@ void dwc3_remove_requests(struct dwc3 *dwc, struct dwc3_ep *dep, int status)
while (!list_empty(&dep->started_list)) {
req = next_request(&dep->started_list);
+ dwc3_gadget_ep_skip_trbs(dep, req);
dwc3_gadget_giveback(dep, req, status);
@@ -1012,6 +1015,7 @@ void dwc3_remove_requests(struct dwc3 *dwc, struct dwc3_ep *dep, int status)
while (!list_empty(&dep->cancelled_list)) {
req = next_request(&dep->cancelled_list);
+ dwc3_gadget_ep_skip_trbs(dep, req);
dwc3_gadget_giveback(dep, req, status);