Messages in this thread | | | Date | Tue, 23 Aug 2022 15:06:17 -0700 | Subject | Re: [PATCH 2/5] usb: gadget: Add function wakeup support | From | Elson Serrao <> |
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On 8/22/2022 6:01 PM, Thinh Nguyen wrote: > On Thu, Aug 18, 2022 at 11:17:24AM -0700, Elson Serrao wrote: >> >> >> 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 > > We don't want to poll that many times. We shouldn't depend on the delay > of a register read for polling interval. Can't we just sleep in between > interval at a reasonable interval. >
Sleeping is not an option as the upper layers (those beyond function/composite layer) may transmit data with a lock held.
I ran into below BUG when remote wakeup was triggered via a ping() command and attempted sleep while polling
[ 88.676789][ T392] BUG: scheduling while atomic [ 88.900112][ T392] Call trace: <snip> [ 88.912760][ T392] __schedule_bug+0x90/0x188 [ 88.917335][ T392] __schedule+0x714/0xb4c [ 88.930568][ T392] schedule+0x110/0x204 [ 88.943620][ T392] schedule_timeout+0x94/0x134 [ 88.948371][ T392] __dwc3_gadget_wakeup+0x1ac/0x558 [ 88.953558][ T392] dwc3_gadget_wakeup+0x3c/0x8c [ 88.958388][ T392] usb_gadget_wakeup+0x54/0x1a8 [ 88.963222][ T392] eth_start_xmit+0x130/0x830 [ 88.967876][ T392] xmit_one+0xf0/0x364 [ 88.971913][ T392] sch_direct_xmit+0x188/0x3e4 [ 88.976663][ T392] __dev_xmit_skb+0x480/0x984 [ 88.981319][ T392] __dev_queue_xmit+0x2d4/0x7d8 [ 88.986151][ T392] neigh_resolve_output+0x208/0x2f0 <snip>
The above experiment was done by removing spin_locks if any in the wakeup() path of function/composite/controller drivers. It is running in atomic context due to the lock held by linux networking stack/generic packet scheduler.
So below are the only two approaches I can think of for dwc3_gadget_wakeup() API
1.)Polling based approach: We poll until the link comes up. But cannot sleep while polling due to above reasons.
2.)Interrupt based approach (the patches being discussed currently): When a remote wakeup is triggered enable link state interrupts and return right away. The function/composite drivers are later notified about the ON event via resume callback (in a similar way how we notify U3 to composite layer via suspend callback).
Please let me know if there is any alternate way that you can think of here.
>>> >>>> >>>> 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 > > Holding a spin_lock for this long is not reasonable. We only need to > lock when setting link recovery request but not while polling for DSTS > and waiting for the link to go up. > > BR, > Thinh > >> >> 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
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