linux-kernel - Re: [PATCH v2 1/5] usb: gadget: Add remote wakeup capable flag

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Date:   Mon, 23 Jan 2023 19:33:20 +0000
From:   Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To:     Alan Stern <stern@...land.harvard.edu>
CC:     Elson Serrao <quic_eserrao@...cinc.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 v2 1/5] usb: gadget: Add remote wakeup capable flag

On Sat, Jan 21, 2023, Thinh Nguyen wrote:
> On Fri, Jan 20, 2023, Alan Stern wrote:
> > On Sat, Jan 21, 2023 at 02:02:36AM +0000, Thinh Nguyen wrote:
> > > On Fri, Jan 20, 2023, Alan Stern wrote:
> > > > A UDC design might have multiple versions, some supporting remote wakeup 
> > > > and others not.  But drivers generally use a single static 
> > > > usb_gadget_ops structure, and they don't modify it at runtime to account 
> > > > for hardware differences.  So if a single driver controls those multiple 
> > > > versions, you can't rely on the presence of gadget->ops->wakeup to 
> > > > indicate whether there actually is hardware remote wakeup support.
> > > > 
> > > > Ideally, the usb_gadget structure should have a wakeup_capable flag 
> > > > which the UDC driver would set appropriately (probably during its probe 
> > > > routine).
> > > > 
> > > 
> > > I was thinking that it can be handled by the
> > > usb_gadget_enable_remote_wakeup() so we can do away with the
> > > wakeup_capable flag.
> > 
> > usb_gadget_enable_remote_wakeup() gets called when the gadget or 
> > function is suspended, right?  But a gadget driver may want to know long 
> > before that whether the UDC supports remote wakeup, in order to set up 
> > its config descriptor correctly.
> > 
> 
> No, this is to be called during set configuration. If the configuration
> doesn't support remote wakeup, the device should not be able to send
> remote wakeup.
> 

On second thought, you're right about the descriptor. It's better to
warn and prevent the remote wakeup bit from being set in the descriptor
if the UDC doesn't support remote wakeup. Warning the user at set
configuration is too late.

So, we need both rw_capable flag and usb_gadget_enable_remote_wakeup().

Thanks,
Thinh