Hotplug not working for USB 3.1 docking station on Dell XPS 13

Hotplug not working for USB 3.1 docking station on Dell XPS 13

[Richard van der Hoff]

Hi,

I'm having problems with a Plugable USB-C docking station, with my 
laptop, a Dell XPS 13 (9350). If the docking station is plugged in at 
boot, it works correctly; however, when I hotplug it after boot, the USB 
devices are not detected until I force a rescan of the PCI bus.

I have also raised this as a bug at 
https://bugzilla.kernel.org/show_bug.cgi?id=151261. There are some dmesg 
traces and lspci logs attached to that bug.

The lspci output suggests a chain of three PCI bridges (00:1c.0, 
01:00.0, 02:02.0) before a PCI->USB bridge (05:00.0 or 39:00.0). If I 
boot without the docking station plugged in, 01:00.0 and beyond do not 
appear, and nor do they appear on hotplug. However, if I hotplug the 
docking station and then force a rescan with "echo 1 > 
/sys/bus/pci/devices/0000:00:1c.0/rescan", the PCI bridges and USB 
devices appear and spring into life.

My impression, and feedback from the linux-usb mailing list, is 
therefore that this is a PCI hotplug problem.

There are some ACPI errors in the dmesg at hotplug but they don't really 
mean much to me.

I'd really appreciate any suggestions on how to proceed with this.

Thanks

Richard van der Hoff

Re: Hotplug not working for USB 3.1 docking station on Dell XPS 13

[Bjorn Helgaas]

[+cc Andreas, linux-kernel]

On Wed, Aug 24, 2016 at 05:40:53PM +0100, Richard van der Hoff wrote:
> I'm having problems with a Plugable USB-C docking station, with my
> laptop, a Dell XPS 13 (9350). If the docking station is plugged in
> at boot, it works correctly; however, when I hotplug it after boot,
> the USB devices are not detected until I force a rescan of the PCI
> bus.
> 
> I have also raised this as a bug at
> https://bugzilla.kernel.org/show_bug.cgi?id=151261. There are some
> dmesg traces and lspci logs attached to that bug.
> 
> The lspci output suggests a chain of three PCI bridges (00:1c.0,
> 01:00.0, 02:02.0) before a PCI->USB bridge (05:00.0 or 39:00.0). If
> I boot without the docking station plugged in, 01:00.0 and beyond do
> not appear, and nor do they appear on hotplug. However, if I hotplug
> the docking station and then force a rescan with "echo 1 >
> /sys/bus/pci/devices/0000:00:1c.0/rescan", the PCI bridges and USB
> devices appear and spring into life.
> 
> My impression, and feedback from the linux-usb mailing list, is
> therefore that this is a PCI hotplug problem.

Please include a pointer to the linux-usb analysis.

> There are some ACPI errors in the dmesg at hotplug but they don't
> really mean much to me.

>From https://bugzilla.kernel.org/attachment.cgi?id=227331 (dock
present at boot):

  00:1c.0 PCI bridge: Intel Device 9d10 (PCIe Root Port)
    Bus: primary=00, secondary=01, subordinate=39
  01:00.0 PCI bridge: Intel Device 1576 (Alpine Ridge Thunderbolt)
    Bus: primary=01, secondary=02, subordinate=05
  02:02.0 PCI bridge: Intel Device 1576 (Alpine Ridge Thunderbolt)
    Bus: primary=02, secondary=05, subordinate=05
  05:00.0 USB controller: Intel Device 15b5 (Alpine Ridge USB 3.1)

01:00.0 and 02:02.0 are ports of a Thunderbolt switch.  Since they
don't appear when you boot without the docking station, the dock
connection must be between 00:1c.0 and 01:00.0.

>From the kernel's point of view, plugging in the dock would be a PCIe
hotplug event on 00:1c.0 that should be handled by pciehp.  Do you
have pciehp included in your kernel?  

Bjorn

Re: Hotplug not working for USB 3.1 docking station on Dell XPS 13

[Richard van der Hoff]

On 24/08/16 21:15, Bjorn Helgaas wrote:
> [+cc Andreas, linux-kernel]
>
> On Wed, Aug 24, 2016 at 05:40:53PM +0100, Richard van der Hoff wrote:
>> I'm having problems with a Plugable USB-C docking station, with my
>> laptop, a Dell XPS 13 (9350).  ...
>> My impression, and feedback from the linux-usb mailing list, is
>> therefore that this is a PCI hotplug problem.
>
> Please include a pointer to the linux-usb analysis.

For the record, here: 
http://marc.info/?l=linux-usb&m=147204147226274&w=2, however:

>  From the kernel's point of view, plugging in the dock would be a PCIe
> hotplug event on 00:1c.0 that should be handled by pciehp.  Do you
> have pciehp included in your kernel?

I did not; now I do, and it works perfectly. Thank you! A case of it 
being incredibly obvious once you know what you are looking for.

> 01:00.0 and 02:02.0 are ports of a Thunderbolt switch. Since they
> don't appear when you boot without the docking station, the dock
> connection must be between 00:1c.0 and 01:00.0.

This would strike me as surprising - the dock bills itself as a USB-C 
device, and the Thunderbolt devices look like they are part of the 
laptop chipset to me. To me it seems more likely that those PCI devices 
are part of the laptop, but are powered down until something is plugged 
into the USB-C port.

Still, this is splitting hairs; the point is that they are hotplugged 
PCI devices which need pciehp.

Thanks again for the pointer.

Best regards

Richard

Re: Hotplug not working for USB 3.1 docking station on Dell XPS 13

[Bjorn Helgaas]

On Thu, Aug 25, 2016 at 01:13:27PM +0100, Richard van der Hoff wrote:
> On 24/08/16 21:15, Bjorn Helgaas wrote:
> >[+cc Andreas, linux-kernel]
> >
> >On Wed, Aug 24, 2016 at 05:40:53PM +0100, Richard van der Hoff wrote:
> >>I'm having problems with a Plugable USB-C docking station, with my
> >>laptop, a Dell XPS 13 (9350).  ...
> >>My impression, and feedback from the linux-usb mailing list, is
> >>therefore that this is a PCI hotplug problem.
> >
> >Please include a pointer to the linux-usb analysis.
> 
> For the record, here:
> http://marc.info/?l=linux-usb&m=147204147226274&w=2, however:

Huh, turns out you and Oliver were right about it being a PCI problem.
But it would have saved some time to know how Oliver decided that.
Otherwise it's just "he said, she said."

> > From the kernel's point of view, plugging in the dock would be a PCIe
> >hotplug event on 00:1c.0 that should be handled by pciehp.  Do you
> >have pciehp included in your kernel?
> 
> I did not; now I do, and it works perfectly. Thank you! A case of it
> being incredibly obvious once you know what you are looking for.
> 
> >01:00.0 and 02:02.0 are ports of a Thunderbolt switch. Since they
> >don't appear when you boot without the docking station, the dock
> >connection must be between 00:1c.0 and 01:00.0.
> 
> This would strike me as surprising - the dock bills itself as a
> USB-C device, and the Thunderbolt devices look like they are part of
> the laptop chipset to me. To me it seems more likely that those PCI
> devices are part of the laptop, but are powered down until something
> is plugged into the USB-C port.

Yep, I agree, that sounds like a better theory.  Glad it's working
now.

Bjorn