Re: [PATCH v5 5/5] usb: host: enable sideband transfer during system sleep

[Alan Stern]

On Tue, Oct 15, 2024 at 11:56:00AM +0800, Guan-Yu Lin wrote:
> On Mon, Oct 14, 2024 at 11:56 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> > I'm not so sure about this.  By returning early, you prevent the drivers
> > bound to this device from suspending.  But they can't operate properly
> > when the system is in a low-power mode.  Won't that cause problems?
> >
> > Maybe this really belongs in usb_suspend_device(), and its counterpart
> > belongs in usb_resume_device().
> >
> 
> To my understanding, after the system is suspended, the USB driver
> will do nothing as the main processor has been suspended. May I check
> what forms of low power mode and operation we are discussing here?

S3 suspend.  You are right that the driver will do nothing while the
CPU is suspended.  But what about the times before and after that,
while the suspend and resume procedures are underway?  The driver
needs to be told to cancel any ongoing transfers while the system
suspends and then restart them while the system resumes.

> usb_suspend_device() did close the required port/bus to maintain usb
> transfer.

I don't know what you mean by that.

>  However, there are additional usb_hcd_flush_endpoint() calls
> aside from usb_suspend_device(). Maybe we should consider not doing
> those also since some of the endpoints are now handled by the
> sideband.

Those calls should not be skipped.  If the sideband controller wants
to handle the endpoints on its own, it can go right ahead.  The main
controller and the USB core need to know that they shouldn't use the
endpoint while the system is suspending.

Alan Stern