Re: [PATCH] USB: Fix the issue of task recovery failure caused by USB status when S4 wakes up

[duanchenghao]

在 2024-10-10星期四的 10:21 -0400，Alan Stern写道：
> On Thu, Oct 10, 2024 at 01:59:08PM +0800, duanchenghao wrote:
> > Hi Alan,
> 
> > Thank you very much for your evaluation of the scheme. I have a
> > question regarding why the set_bit operation for the
> > HCD_FLAG_WAKEUP_PENDING flag is performed in the top half of an
> > interrupt handler, while the clear_bit operation is done in the
> > bottom
> > half. This seems to contradict conventional practices. The issue is
> > not
> > limited to S4; if other processes freeze the work queue in the
> > bottom
> > half, the same problem may arise.
> 
> The flag is treated this way because that's what it means: A wakeup
> is 
> pending.  The kernel first learns about the wakeup when it receives
> the 
> wakeup interrupt from the host controller, so that's when it sets the
> flag -- in the top half of the interrupt handler.  The wakeup
> procedure 
> isn't complete until the root hub has been resumed, so the flag
> remains 
> set until that resume is finished, in the bottom half.
> 
> You say "the same problem may arise", but I don't think it is a
> problem.  
> If the system is about to suspend the host controller with wakeups 
> enabled, and a wakeup request has already been received but the
> system 
> has not yet finished acting on it, then the suspend _should_ fail.  
> After all, if the wakeup interrupt had arrived just after the host 
> controller was suspended rather than just before, it would have
> caused 
> the host controller to be resumed right away -- with exactly the same
> effect as if the controller had never been suspended in the first
> place.
> 
> > The solution you described below should be able to resolve the
> > current
> > S4 issue, but for now, we haven't identified any other scenarios
> > that
> > require the same operation.
> 
> Perhaps because there aren't any.
> 
> >  Based on my understanding, the USB device
> > is woken up in the bottom half of the interrupt,
> 
> You are failing to distinguish between the host controller and the
> root 
> hub.  The host controller (which is a PCI device on your system, not
> a 
> USB device) is woken up in the top half of the interrupt handler. 
> The 
> root hub (which is a virtual USB device) is woken up in the bottom
> half.  
> Both operations have to occur before the wakeup can be considered
> fully 
> complete.
> 
> >  and both the set_bit
> > and clear_bit operations for the HCD_FLAG_WAKEUP_PENDING flag
> > should be
> > executed within the same thread in the bottom half. May I ask if
> > there
> > are any other reasons why the set_bit is executed in the top half?
> 
> Because the root hub's wakeup becomes pending when the host
> controller 
> is resumed, in the top half.
> 
> Alan Stern
> 

Hi Alan,

I roughly understand now.

In your previous email, you mentioned assigning a value to do_wakeup
based on the judgment of PMSG in suspend_common, but there is no
parameter passing of PMSG in suspend_common.

Do you mean using the global parameter pm_transition.event for the
judgment?

Thanks
Duanchenghao