Re: [PATCH] usb: gadget: f_uac2: fixup feedback endpoint stop

Jerome Brunet wrote:

On Wed 25 Aug 2021 at 21:42, Thinh Nguyen [off-list ref] wrote:

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Ferry Toth wrote:

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Hi,

Op 25-08-2021 om 11:21 schreef Jerome Brunet:

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When the uac2 function is stopped, there seems to be an issue with some
platforms (Intel Merrifield at least)

The issue isn't hardware specific.

While the actual bug isnt, the report was (given the issue did not show
up during initial testing but did show on Ferry's HW)
Merely citing the bug report from Ferry here

Ok

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BUG: kernel NULL pointer dereference, address: 0000000000000008
...
RIP: 0010:dwc3_gadget_del_and_unmap_request+0x19/0xe0
...
Call Trace:
  dwc3_remove_requests.constprop.0+0x12f/0x170
  __dwc3_gadget_ep_disable+0x7a/0x160
  dwc3_gadget_ep_disable+0x3d/0xd0
  usb_ep_disable+0x1c/0x70
  u_audio_stop_capture+0x79/0x120 [u_audio]
  afunc_set_alt+0x73/0x80 [usb_f_uac2]
  composite_setup+0x224/0x1b90 [libcomposite]

The issue happens only when the gadget is using the sync type "async",
not
"adaptive". This indicates that problem is likely coming from the
feedback
endpoint, which is only used with async synchronization mode.

This does not describe the actual problem. The problem is that the
usb_ep_dequeue() can be an asynchronous call, and we can't free the
request until its completion (from cancellation).

Indeed. I was not sure at the time.

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Update the feedback endpoint free function to release the endpoint the
same
way it is done for the data endpoint.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
---

  Hi Ferry,

  Would you mind trying this before reverting the whole thing ?
  The HW I have did not show the issue so far so I can't really check
  if it helps. Hopefully, it does ...

Tested this evening and confirming that this resolves my issue. I can't
say much about the code itself, maybe Thinh?

Sure. I can take a look.

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Would be great if we could get this in instead of reverting the series.

Tested-by:  Ferry Toth <redacted> (dwc3 / Intel Merrifield)

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  drivers/usb/gadget/function/u_audio.c | 15 +++++++++++----
  1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/gadget/function/u_audio.c

b/drivers/usb/gadget/function/u_audio.c
index 018dd0978995..63d9340f008e 100644

--- a/drivers/usb/gadget/function/u_audio.c
+++ b/drivers/usb/gadget/function/u_audio.c

@@ -230,7 +230,13 @@ static void u_audio_iso_fback_complete(struct

usb_ep *ep,
      int status = req->status;
        /* i/f shutting down */
-    if (!prm->fb_ep_enabled || req->status == -ESHUTDOWN)
+    if (!prm->fb_ep_enabled) {

prm->fb_ep_enabled is not protected. Potential race problem here?

Given how the variable is used, I don't think so.
Could you please detail ?

I'm thinking of this scenario:

Since free_ep_fback() is called in a separate thread,
u_audio_iso_fback_complete() can come in the middle after
prm->fb_ep_enabled is cleared but before the usb_ep_dequeue(). So the
request may be freed before being accessed again in usb_ep_dequeue().

(I don't think this is really related to the current problem though)

It's related to how we go about to solve the problem right?

Can we just check for the req->status for -ECONNRESET and -ESHUTDOWN and
free the request? Those 2 statuses should be for when the request is
cancelled and when the endpoint is disabled.

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+        kfree(req->buf);
+        usb_ep_free_request(ep, req);
+        return;
+    }
+
+    if (req->status == -ESHUTDOWN)
          return;
        /*

@@ -421,9 +427,10 @@ static inline void free_ep_fback(struct

uac_rtd_params *prm, struct usb_ep *ep)
      prm->fb_ep_enabled = false;
        if (prm->req_fback) {
-        usb_ep_dequeue(ep, prm->req_fback);
-        kfree(prm->req_fback->buf);
-        usb_ep_free_request(ep, prm->req_fback);
+        if (usb_ep_dequeue(ep, prm->req_fback)) {
+            kfree(prm->req_fback->buf);
+            usb_ep_free_request(ep, prm->req_fback);
+        }
          prm->req_fback = NULL;
      }
  

On a separate note, I notice that f_uac2 only queues a single feedback
request at a time for isoc endpoint? Even though the interval is 1ms,
this will easily cause data drop.

Also, you're ignoring other request error status and still processing
bogus data on request completion? That doesn't seem right.

Gadget is sendind the feedback data, not processing it. Every data sent

Ah ok.. I missed that it's IN request.

is OK.  Yes, packets can be missed with the current implementation,
meaning the feedback value is not reported as often as initially
intended. On slower HW, packets are also missed with 2 requests queued,

2 is still too low. From our HW testing, minimum should be 4 to minimize
data loss at 1ms interval.

not only on the feedback endpoint but also on the playback
endpoint. Picking the approriate value is not straight forward. For the
feedback endpoint it isn't big deal because, according to the spec, if
the feedback is not sent, the host shall assume the value hasn't
changed. This why the whole thing works as it is.

Why do this when we can avoid it. We know that there will be data drop
with the way it's implemented now. There's no option to adjust the
number of feedback requests either.

I admit things still aren't perfect, but there is progress ...

Thanks for the updates to audio/f_uac2. It's been a long time since
anyone working on it.

BR,
Thinh