RE: [PATCH 2/2] usb: host: ehci-platform: add a quirk to avoid stuck

[Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>]

Hi Alan,

> From: Alan Stern, Sent: Wednesday, January 22, 2020 11:59 PM
> 
> On Wed, 22 Jan 2020, Yoshihiro Shimoda wrote:
> 
> > > Okay, now I understand.  I misread the code in the original patch.
> > > But now it looks like the code does roughly this:
> > >
> > > Timer routine:	if (ehci_platform_quirk_poll_check_condition(ehci))
> > > 			schedule_work();
> > >
> > > Work routine:	usleep_range(4000, 8000);
> > > 		udelay(10);
> > > 		if (!ehci_platform_quirk_poll_check_condition(ehci))
> > > 			return;
> > > 		udelay(10);
> > > 		if (!ehci_platform_quirk_poll_check_condition(ehci))
> > > 			return;
> > > 		ehci_platform_quirk_poll_rebind_companion(ehci);
> > >
> > > So there are three calls to quirk_poll_check_condition, with 4 - 8 ms
> > > between the first and second, and 10 us between the second and third.
> > > Do you really need to have this combination of a long delay followed by
> > > a short delay?  Wouldn't two check_condition calls with only one delay
> > > be good enough?
> >
> > I had implemented this code by using hardware team's suggestion without
> > any doubt. So, I asked hardware team about this combination of delays.
> > The hardware team said this combination can reduce misdetection ratio
> > from noise and so on. They also said we can wait single 5 ms instead
> > this combination (but this cannot reduce misdetection ratio).
> 
> Sure, the more times you delay and recheck, the better the error rate.
> But you don't want to go too far.

You're correct. However, my mind is changed a little...
I would like to remain rechecking as the same as before.

> > So, now I'm thinking that the following process (single wait) is
> > enough and it can improve readability. But, what do you think?
> >
> > Timer routine:	if (ehci_platform_quirk_poll_check_condition(ehci))
> >  			schedule_delayed_work(5 ms);
> >
> > Delayed work routine:
> > 		if (!ehci_platform_quirk_poll_check_condition(ehci))
> >  			return;
> >  		ehci_platform_quirk_poll_rebind_companion(ehci);
> 
> That looks good to me.

Thank you for your feedback! I'll submit v2 patch soon.

Best regards,
Yoshihiro Shimoda

> Alan Stern