Re: USB-C device hotplug issue

[Mathias Nyman]

On 26.10.2018 17:07, Alan Stern wrote:
> On Fri, 26 Oct 2018, Dennis Wassenberg wrote:
>
> > > > --- a/drivers/usb/core/hub.c
> > > > +++ b/drivers/usb/core/hub.c
> > > > @@ -2815,7 +2815,9 @@ static int hub_port_reset(struct usb_hub *hub, int port1,
> > > >   					USB_PORT_FEAT_C_BH_PORT_RESET);
> > > >   			usb_clear_port_feature(hub->hdev, port1,
> > > >   					USB_PORT_FEAT_C_PORT_LINK_STATE);
> > > > -			usb_clear_port_feature(hub->hdev, port1,
> > > > +
> > > > +			if (!warm)
> > > > +				usb_clear_port_feature(hub->hdev, port1,
> > > >   					USB_PORT_FEAT_C_CONNECTION);
> > > >   
> > > >   			/*
> > >
> > > The key fact is that connection events can get lost if they happen to
> > > occur during a port reset.
> > Yes.
> > > I'm not entirely certain of the logic here, but it looks like the
> > > correct test to add should be "if (udev != NULL)", not "if (!warm)".
> > > Perhaps Mathias can confirm this

Sorry about the late response, got distracted while performing git
archeology.

"if (udev != NULL)" would seem more reasonable.

Logs show that clearing the FEAT_C_CONNECTION was originally added
after a hot reset failed, and before issuing a warm reset to a SS.Inactive
link.  (see 10d674a USB: When hot reset for USB3 fails, try warm reset.)

Apparently all the change flags needed to be cleared for some specific
host + device combination before issuing a warm reset for the enumeration
to work properly.

The change to always clear FEAT_C_CONNECTION for USB3 was done later in patch:
a24a607 USB: Rip out recursive call on warm port reset.

Motivation was:

"In hub_port_finish_reset, unconditionally clear the connect status
 change (CSC) bit for USB 3.0 hubs when the port reset is done.  If we
 had to issue multiple warm resets for a device, that bit may have been
 set if the device went into SS.Inactive and then was successfully warm
 reset."

> > I don't know if clearing the USB_PORT_FEAT_C_CONNECTION bit is
> > correct in case of a non warm reset. In my case I always observed a
> > warm reset because of the link state change.
> > Thats why I checked the warm variable to not change the behavoir for
> > cases a didn't checked for the first shot.
>
> (The syntax of that last sentence is pretty mangled; I can't understand
> it.)
>
> Think of it this way:
>
> 	If a device was not attached before the reset, we don't want
> 	to clear the connect-change status because then we wouldn't
> 	recognize a device that was plugged in during the reset.
>
> 	If a device was attached before the reset, we don't want any
> 	connect-change status which might be provoked by the reset to
> 	last, because we don't want the core to think that the device
> 	was unplugged and replugged when all that happened was a reset.
>
> So the important criterion is whether or not a device was attached to
> the port when the reset started.  It's something of a coincidence that
> you only observe warm resets when there's nothing attached.
>
> > During the first run of usb_hub_reset the udev is NULL because in
> > this situation the device is not attached logically.
> >
> > [  112.889810] usb 4-1-port1: XXX: port_event: portstatus: 0x2c0, portchange: 0x40!
> > [  113.201192] usb 4-1-port1: XXX: hub_port_reset: udev:            (nil)!
> > [  113.201198] usb 4-1-port1: XXX: hub_port_reset (not clearing USB_PORT_FEAT_C_CONNECTION): 0x203, portchange: 0x1!
> > [  113.253612] usb 4-1-port1: XXX: port_event: portstatus: 0x203, portchange: 0x1!
> > [  113.377208] usb 4-1-port1: XXX: hub_port_reset: udev: ffff88046b302800!
> > [  113.377214] usb 4-1-port1: XXX: hub_port_reset (not clearing USB_PORT_FEAT_C_CONNECTION): 0x203, portchange: 0x0!
> > [  113.429478] usb 4-1.1: new SuperSpeed USB device number 7 using xhci_hcd
> > [  113.442370] usb 4-1.1: New USB device found, idVendor=0781, idProduct=5596
> > [  113.442376] usb 4-1.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
> > [  113.442381] usb 4-1.1: Product: Ultra T C
> > [  113.442385] usb 4-1.1: Manufacturer: SanDisk
> > [  113.442388] usb 4-1.1: SerialNumber: 4C530001131013121031
> >
> > Or maybe we can skip clearing the USB_PORT_FEAT_C_CONNECTION bit in
> > case of hub_port_reset completely without any other check?
>
> See above.

Checking for udev sounds reasonable to me.
Not sure if we should worry about the specific host+device combo that needed flags
cleared before warm reset. See patch:

10d674a USB: When hot reset for USB3 fails, try warm reset.
https://marc.info/?l=linux-usb&m=131603549603799&w=2

-Mathias