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

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Date:   Fri, 10 Feb 2023 02:27:28 +0000
From:   Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To:     Elson Serrao <quic_eserrao@...cinc.com>
CC:     Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        "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 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