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

[Thinh Nguyen]

On 8/9/2022, Elson Serrao wrote:
> 
> 
> On 8/4/2022 6:26 PM, Thinh Nguyen wrote:
>> On 8/4/2022, Elson Serrao wrote:
>>>
>>>
>>> On 8/2/2022 4:51 PM, Thinh Nguyen wrote:
>>>> On 8/2/2022, Elson Roy Serrao wrote:
>>>>> An interface which is in function suspend state has to send a function
>>>>> wakeup notification to the host in case it needs to initate any data
>>>>> transfer. One notable difference between this and the existing remote
>>>>> wakeup mechanism is that this can be called per-interface, and a UDC
>>>>> would need to know the particular interface number to convey in its
>>>>> Device Notification transaction packet.  Hence, we need to introduce
>>>>> a new callback in the gadget_ops structure that UDC device drivers
>>>>> can implement.  Similarly add a convenience function in the composite
>>>>> driver which function drivers can call. Add support to handle such
>>>>> requests in the composite layer and invoke the gadget op.
>>>>
>>>> Sending the function wake notification should be done in the controller
>>>> driver. The controller driver knows when is the proper link state
>>>> (U0/ON) the device is in and would notify the host then.
>>>>
>>>> What we need to add in the usb_gadget is whether the device is remote
>>>> wakeup capable. Something like a flag usb_gadget->rw_capable.
>>>>
>>>> We would also need some functions like
>>>> usb_gadget_enable_remote_wakeup()
>>>> and usb_gadget_disable_remote_wakeup() for the gadget driver to notify
>>>> the controller driver when it checks against USB_CONFIG_ATT_WAKEUP in
>>>> the bmAttributes configuration.
>>>>
>>>> BR,
>>>> Thinh
>>>
>>>
>>> If we handle this in controller driver, then it would fail to get the
>>> right interface id when multiple functions have to send function wake
>>> notification. As per USB3.0 spec (below snippets) a function can be
>>> independently placed into function suspend state within a composite
>>> device and each function in function suspend state has to send a
>>> function wake notification to exit.
>>>
>>> USB 3.0 Spec Section 9.2.5.3
>>> "A function may be placed into Function Suspend independently of other
>>> functions within a composite device"
>>>
>>> USB 3.0 Spec Section 9.2.5.4
>>> "A function may signal that it wants to exit from Function Suspend by
>>> sending a Function Wake Notification to the host if it is enabled for
>>> function remote wakeup. This applies to single function devices as
>>> well as multiple function ( i.e., composite) devices. If the link is in
>>> a non-U0 state, then the device must transition the link to U0 prior
>>> to sending the remote wake message. If a remote wake event occurs in
>>> multiple functions, each function shall send a Function Wake
>>> Notification"
>>>
>>
>> Ok, so the issue here is adding the ability to pass the interface number
>> to the controller driver when sending the device notification function
>> wakeup right? Sounds like the callback should be
>> send_wakeup_notification(gadget, func_id) instead.
>>
>> As for remote wakeup, the spec states that "If remote wake event occurs
>> in multiple functions, each function shall send a Function Wake
>> Notification."
>>
>> The SET_FEATURE(FUNCTION_SUSPEND) does not necessarily mean the host
>> will put the device in Suspend State for a remote wake event to occur.
>> It only places the function in Function Suspend. However often the host
>> will put the device in suspend after this. The dwc3 driver can track if
>> the host puts the device in suspend state and what interfaces are armed
>> for remote wakeup. If a remote wakeup event occurs, the dwc3 driver can
>> send Function Wake Notification for each function armed with remote
>> wakeup.
>>
>> Please correct me if I'm wrong.
>>
>> Also, make sure that device remote wakeup will still work for highspeed
>> (not function remote wakeup). I see this check which doesn't look right
>> in one of your patches:
>> +    if (g->speed < USB_SPEED_SUPER && !dwc->is_remote_wakeup_enabled)
>> +        dev_err(dwc->dev, "%s:remote wakeup not supported\n", __func__);
>> +        ret =  -EPERM;
>> +        goto out;
>> +    }
>>
>> Thanks,
>> Thinh
>>
> 
> For superspeed capable devices, when a function is in suspend state and
> wants to
> initiate a resume, it has to send a function wake notification to the
> host irrespective
> of whether the device is in SUSPEND or not. Like you mentioned the
> device need not be in
> suspend state when a function is suspended. If the device is in suspend,
> then first the
> controller driver has to transition the link to U0 state before sending
> function wake notification.

Was I incorrect? I'm not clear on the point of reiteration above.

> Note that the DEVICE_REMOTE_WAKEUP feature is ignored for super-speed
> devices and they

We're still talking about Enhanced Super Speed here.

> are by default remote wakeup capable if any function within the device
> is armed for
> function remote wakeup.

What you're saying is if the host arms the function for remote wakeup,
then the device is remote capable.

However, the important point here is that the host only arms for remote
wakeup _if_ the device is remote wakeup capable. That needs to be checked.

> 
> So in my current implementation when the host sends a function suspend
> SET_FEATURE(FUNCTION_SUSPEND),
> the device delegates it to the respective function driver. There we
> inspect if it is capable
> of initiating a function remote wakeup. If it is, then when a remote
> wakeup event
> occurs (in my current implementation when TCP/IP layer wants to send
> data to the host. patch#5) then
> we trigger a function wakeup by calling usb_gadget_func_wakeup(gadget,
> id) callback. Controller driver then
> checks if the device is in suspend or not. If it is in suspend, it first
> brings the device to U0 state

"brings the device to U0 state" means the device initiates remote wakeup
here.

> and then sends a function wake notification (via
> dwc3_send_gadget_generic_command() API) only after an

So now the dwc3 tracks which interface(s) were armed for remote here?

I don't recall seeing it in your patches. Did you handle and send device
notification for all the functions armed with remote wakeup after device
wakeup?


> U0 event has occurred. If the device is not in suspend then it directly
> sends function wake notification
> to the host. Once the host receives the function wake notification it
> sends a SET_FEATURE(FUNCTION_SUSPEND)
> with suspend bit (BIT 0) reset to signal function resume. The controller
> driver upon receiving this packet
> delegates to the respective function driver. Note that at this point the
> device is in U0 state but some other

We can't assume that the device is in U0 state. There's also no
mechanism in your change to know that either.

> function within the device may still be in suspend state (if more than
> one function was put to suspend state).
> So the only way to exit from function suspend is via function resume
> which is independent of device suspend/resume.
> 
> Also the task of finding the interface id is done by composite driver
> because most function drivers have
> a transport layer and this layer is the one responsible for issuing a
> function remote wakeup and
> this has no direct reference to interface id. For example u_ether
> transport layer can have either f_ecm or f_rndis
> as its underlying channel and u_ether has no knowledge of the interface
> id/function driver it is using.
> 

> For high speed devices there is no concept of function suspend and there
> is only device suspend. The ability
> of a device to send a remote wakeup to exit from suspend is dictated by
> DEVICE_REMOTE_WAKEUP feature selector.
> The below snippet controls this aspect and sends remote wakeup for high
> speed devices only if they are remote wakeup capable.
> dwc->is_remote_wakeup_enabled flag is set when DEVICE_REMOTE_WAKEUP is
> received.
>

The flag "is_remote_wakeup_enabled" implies that it applies for both
device remote wakeup and function remote wakeup. If it only meant for
function remote wakeup, then rename it. But I think you can use the same
flag for both scenarios.


> +    if (g->speed < USB_SPEED_SUPER && !dwc->is_remote_wakeup_enabled)
> +        dev_err(dwc->dev, "%s:remote wakeup not supported\n", __func__);
> +        ret =  -EPERM;

Also, don't use -EPERM. Use -EINVAL.

> +        goto out;
> +    }
> 
> Please let me know your thoughts on this approach. I will address your
> other comments and rectify the patches accordingly.
> 


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).

To be able to do 3), you can teach the composite layer _when_ to send
device notification function wake and for what functions. This can be
retry sending the notification until send_wakeup_notification() succeed?

I suggested to do that in dwc3 driver to avoid having to add the logic
in composite layer as I think it is simpler in dwc3. However, the
downside is that other UDCs have to handle it like dwc3 also.

Now that I think about it again, it maybe better to do it in the
composite driver for the long run. If you want to handle this in the
composite layer, please document and design the mechanism to handle all
the points above.

Thanks,
Thinh