Re: [PATCH 2/5] usb: gadget: Add function wakeup support

[Elson Serrao]

On 8/16/2022 4:51 PM, Thinh Nguyen wrote:
> On 8/16/2022, Elson Serrao wrote:
> > On 8/12/2022 5:46 PM, Thinh Nguyen wrote:
> > > On 8/11/2022, Thinh Nguyen wrote:
> > > > On 8/11/2022, Thinh Nguyen wrote:
> > > > > On 8/11/2022, Elson Serrao wrote:
> > > > > > On 8/9/2022 6:08 PM, Thinh Nguyen wrote:
> > > > >
> > > > > <snip>
> > > > >
> > > > >
> > > > > > > To summarize the points:
> > > > > > >
> > > > > > > 1) The host only arms function remote wakeup if the device is
> > > > > > > capable of
> > > > > > > remote wakeup (check USB_CONFIG_ATT_WAKEUP in bmAttributes and
> > > > > > > hardware
> > > > > > > capability)
> > > > > > >
> > > > > > > 2) If the device is in suspend, the device can do remote wakeup
> > > > > > > (through
> > > > > > > LFPS handshake) if the function is armed for remote wakeup (through
> > > > > > > SET_FEATURE(FUNC_SUSPEND)).
> > > > > > >
> > > > > > > 3) If the link transitions to U0 after the device triggering a remote
> > > > > > > wakeup, the device will also send device notification function
> > > > > > > wake for
> > > > > > > all the interfaces armed with remote wakeup.
> > > > > > >
> > > > > > > 4) If the device is not in suspend, the device can send device
> > > > > > > notification function wake if it's in U0.
> > > > > > >
> > > > > > >
> > > > > > > Now, remote wakeup and function wake device notification are 2
> > > > > > > separate
> > > > > > > operations. We have the usb_gadget_ops->wakeup() for remote wakeup. I
> > > > > > > suggested to maybe add
> > > > > > > usb_gadget_ops->send_wakeup_notification(gadget,
> > > > > > > intf_id) for the device notification. What you did was combining both
> > > > > > > operations in usb_gadget_ops->func_wakeup(). That may only work for
> > > > > > > point 4) (assuming you fix the U0 check), but not point 3).
> > > > > >
> > > > > > Thank you for your feedback and summary. I will rename func_wakeup to
> > > > > > send_wakeup_notification to better align with the approach. The
> > > > > > reason I
> > > > > > have combined remote_wakeup and function wake notification in
> > > > > > usb_gadget_ops->func_wakeup() is because since the implementation
> > > > > > is at
> > > > > > function/composite level it has no knowledge on the link state. So I
> > > > > > have delegated that task to controller driver to handle the
> > > > > > notification
> > > > > > accordingly. That is do a LFPS handshake first if the device is
> > > > > > suspended and then send notification (explained below). But we can
> > > > > > definitely separate this by adding an additional flag in the composite
> > > > > > layer to set the link state based on the gadget suspend callback
> > > > > > called
> > > > > > when U3 SUSPEND interrupt is received. Let me know if you feel
> > > > > > separating the two is a better approach.
> > > > >
> > > > > The reason I think we need to separate it is because of point 3. As I
> > > > > note earlier, the spec states that "If remote wake event occurs in
> > > > > multiple functions, each function shall send a Function Wake
> > > > > Notification."
> > > > >
> > > > > But if there's no remote wake event, and the host brought the device up
> > > > > instead, then the function suspend state is retained.
> > > > >
> > > > > If we separate these 2 operations, the caller can check whether the
> > > > > operation went through properly. For example, if the wakeup() is
> > > > > initiated properly, but the function wake device notification didn't go
> > > > > through. We would only need to resend the device notification rather
> > > > > than initiate remote wakeup again.
> > > >
> > > > If we don't have to send device notification for other interfaces, we
> > > > can combine the operations here as you did.
> > >
> > > I still think it's better to split up the operations. The way you're
> > > handling it now is not clear.
> > >
> > > If the func_awake() returns -EAGAIN, I'd expect that the remote wake did
> > > not go through and expect user to retry again. But here it does initiate
> > > remote wake, but it just does not send device notification yet. This is
> > > confusing.
> > >
> > > Also, instead of all the function wake handling coming from the function
> > > driver, now we depend on the controller driver to call function resume()
> > > on state change to U0, which will trigger device notification. What
> > > happen if it doesn't call resume(). There's too many dependencies and it
> > > seems fragile.
> > >
> > > I think all this can be handled in the function driver. You can fix the
> > > dwc3 wakeup() and poll for U0/ON state rather than RECOVERY state, which
> > > is what it's supposed to poll.
> >
> > For transitioning from U3 to U0, the current upstream implementation is
> > to poll for U0 state when dwc3_gadget_wakeup() is called and it is a
> > blocking call. (this is a common API for both HS and SS)
> >
> >      /* poll until Link State changes to ON */
> >      retries = 20000;
> >
> >      while (retries--) {
> >          reg = dwc3_readl(dwc->regs, DWC3_DSTS);
> >
> >          /* in HS, means ON */
> >          if (DWC3_DSTS_USBLNKST(reg) == DWC3_LINK_STATE_U0)
> >              break;
> >      }
> >
> > In my experiments I found that certain hosts take longer time to drive
> > HS resume signalling between the remote wakeup and the resume K and this
> > time varies across hosts. Hence the above polling timer is not generic
> > across hosts. So how do we converge on a polling timer value to work
> > across HS/SS and without blocking the system for a long time?
>
> Can't we take the upper limit of both base on experiment? And it
> shouldn't be blocking the whole system.

On the host I was experimenting with, the time it took was around 110ms 
in HS case. That would translate to a retry count of about ~181,000 in 
the above polling loop. Wouldn't that be a very large value for polling?
And not sure if there are other hosts that take even longer time
> > Some data layers like TCP/IP hold a TX lock while sending data (that
> > causes a remote wakeup event) and hence this wakeup() may run in atomic
> > context.
>
> Why hold the lock while waiting for the host to wakeup? The host is
> still inactive. Also, the usb_gadget_wakeup() API doesn't specify that
> it may run in atomic context.

The lock might be held by upper layers who are unaware/independent of 
underlying transport medium. The above TX lock I was referring to was
that held by Linux networking stack. It just pushes out data the same 
way it would when USB is active. It is the function/composite layer 
being aware of the function suspend would now sense this as a remote 
wake event and perform this additional step of bringing out the link 
from u3 and then sending device wakeup notification.

In our current upstream implementation of dwc3_gadget_wakeup() API we 
hold a spinlock as well. But yeah that can be rectified

static int dwc3_gadget_wakeup(struct usb_gadget *g)
{
	struct dwc3		*dwc = gadget_to_dwc(g);
	unsigned long		flags;
	int			ret;

	spin_lock_irqsave(&dwc->lock, flags);
	ret = __dwc3_gadget_wakeup(dwc);
	spin_unlock_irqrestore(&dwc->lock, flags);

	return ret;
}


> > To make this generic across hosts, I had switched to interrupt based
> > approach, enabling link state events and return error value immediately
> > from the dwc3_gadget_wakeup() API after doing a LFPS handshake. But
> > yeah, then we have to rely on the resume callback as an indication that
> > link is transitioned to ON state.
>
> BR,
> Thinh