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    SubjectRe: Regression causes a hang on boot with a Comtrol PCI card
    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.

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