linux-kernel - Re: [PATCH v3 2/5] usb: dwc3: Add remote wakeup handling

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Date:   Thu, 9 Feb 2023 19:43:12 -0800
From:   Elson Serrao <quic_eserrao@...cinc.com>
To:     Thinh Nguyen <Thinh.Nguyen@...opsys.com>
CC:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "balbi@...nel.org" <balbi@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "quic_wcheng@...cinc.com" <quic_wcheng@...cinc.com>,
        "quic_jackp@...cinc.com" <quic_jackp@...cinc.com>
Subject: Re: [PATCH v3 2/5] usb: dwc3: Add remote wakeup handling



On 2/9/2023 6:27 PM, Thinh Nguyen wrote:
> On Thu, Feb 09, 2023, Elson Serrao wrote:
>>
>>
>> On 2/7/2023 6:11 PM, Thinh Nguyen wrote:
>>> On Tue, Feb 07, 2023, Elson Serrao wrote:
>>>>
>>>>
>>>> On 2/7/2023 5:10 PM, Thinh Nguyen wrote:
>>>>> On Tue, Feb 07, 2023, Elson Serrao wrote:
>>>>>> On 2/6/2023 4:48 PM, Thinh Nguyen wrote:
>>>>>>>> +static int __dwc3_gadget_wakeup(struct dwc3 *dwc, bool async)
>>>>>>>>      {
>>>>>>>>      	int			retries;
>>>>>>>> @@ -2296,9 +2309,14 @@ static int __dwc3_gadget_wakeup(struct dwc3 *dwc)
>>>>>>>>      	link_state = DWC3_DSTS_USBLNKST(reg);
>>>>>>>>      	switch (link_state) {
>>>>>>>> +	case DWC3_LINK_STATE_U3:	/* in HS, means SUSPEND */
>>>>>>>
>>>>>>> It's also possible to do remote wakeup in L1 for highspeed.
>>>>>>>
>>>>>>
>>>>>> The rw_configured flag here is in context of triggering remote wakeup from
>>>>>> bus suspend only.
>>>>>>
>>>>>> The remote wakeup setting for l1 in HighSpeed is controlled through LPM
>>>>>> token and overrides/ignores the config desc bmAttributes wakeup bit.
>>>>>>
>>>>>> Section 4.1 of USB2_LinkPowerMangement_ECN[final] spec "The host system sets the Remote Wake Flag parameter in this request to
>>>>>> enable or disable the addressed device
>>>>>> for remote wake from L1. The value of this flag will temporarily (while in
>>>>>> L1) override the current setting of the
>>>>>> Remote Wake feature settable by the standard Set/ClearFeature() commands
>>>>>> defined in Universal Serial Bus Specification, revision 2.0, Chapter 9."
>>>>>>
>>>>>> Please let me know if I am missing something.
>>>>>>
>>>>>
>>>>> It overrides the setting of the SetFeature request, not the device
>>>>> configuration.
>>>>>
>>>>> The rw_configured reflects the user configuration. Whether the host
>>>>> tries to enable the remote wakeup through SetFeature request or LPM
>>>>> token, the device should operate within the user configuration
>>>>> limitation.
>>>>>
>>>>> If the configuration indicates that it doesn't support remote wakeup, we
>>>>> should prevent unexpected behavior from the device. For simplicity, we
>>>>> can just return failure to wakeup for all states.
>>>>>
>>>>> Thanks,
>>>>> Thinh
>>>>
>>>> L1 entry/exit is HW controlled and the remote wakeup is conditional.(Section
>>>> 7.1/Table7.2 of dwc3 data book). Even though we block it from
>>>> SW the l1 exit will still happen from HW point of view.
>>>>
>>>> To correlate the user configuration with LPM token, I experimented by
>>>> disabling the wakeup bit in the bmAtrributes, but I still see remote wakeup
>>>> bit being set in the LPM token. From the observation it seems like there is
>>>
>>> That's because the linux xhci driver enables remote wakeup bit in its
>>> port without regard for the device configuration.
>>>
>>>> no correlation between the wakeup bit in the bmAtrributes and the wakeup bit
>>>> in the LPM token.
>>>>
>>>
>>> The host can bring the device out of L1, that's probably what you saw.
>>> The controller doesn't initiate remote wakeup by itself.
>>>
>>> Thanks,
>>> Thinh
>>
>> Actually it seems the controller is initiating a remote wakeup by itself to
>> exit from l1 when we send a STARTTRANSFER command. I did below experiment
>> when the device was in HighSpeed
>>
> 
> That's driven by the driver telling the controller to initiate remote
> wakeup and not the controller itself. When we send the START_TRANSFER
> command, the driver does remote wakeup so the host would bring the
> device to ON state so that the command can go through.
> 
> However you bring up a good point that if we prevent remote wakeup for
> L1, then we have to delay sending START_TRANSFER command until the host
> initiate resume. This would require additional enhancement to dwc3 to
> handle this scenario. For now, can we ignore this specific case when
> sending START_TRANSFER command and only check for the case when the user
> trigger remote wakeup via gadget->ops->wakeup/func_wakeup.
> 
> Thanks,
> Thinh

Sure. I will upload v4 with the suggested feedback/comments.

Thanks
Elson