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From:   Jesse Hathaway <jesse@mbuki-mvuki.org>
Date:   Mon, 15 Apr 2019 16:47:06 -0500
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Subject: Re: Regression causes a hang on boot with a Comtrol PCI card
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Bjorn Helgaas <helgaas@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
        Mathias Nyman <mathias.nyman@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-usb@vger.kernel.org
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On Sat, Apr 6, 2019 at 10:32 AM Alan Stern <stern@rowland.harvard.edu> wrote:
> Well, at least that's forward progress.  I don't know what pstore is or
> what connection it has to the USB subsystem.  Does the machine hang
> similarly if you boot without the Comtrol PCI card present?

Yes the box boots fine when the Comtrol PCI card is *not* present.

> For that matter, what happens if you remove EHCI from the kernel
> configuration completely?

If I remove USB support, the box still hangs after registering the pstore, but
if I remove pstore support and APEI support from the kernel then the box boots
without issue.

> As for how the PCI card affects the USB handoff, it depends on how the
> BIOS behaves.  Normally the BIOS will take control of all the available
> EHCI controllers during bootup (so that it can use them to communicate
> with a USB keyboard or mouse), including controllers on add-on PCI
> cards as well as those on the motherboard.  When the kernel starts up,
> it tries to take ownership of the controllers away from the BIOS
> (that's the handoff) so that Linux can use them.  However, if the BIOS
> was never tested for handoff of USB controllers on add-on PCI cards, it
> could easily have a bug that would crash the machine.

The Comtrol card provides 32 serial ports, via a breakout box, but it has
no USB functionality, which was why I was surprised that its presence
somehow breaks the USB hand off.