[Bug 237666] repeated messages of "uhub_reattach_port: giving up port reset - device vanished"

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=237666

--- Comment #134 from Mark Millard <marklmi26-fbsd at yahoo.com> ---
(In reply to Mark Millard from comment #133)

The code in xhci_do_command does a doorbell/mailbox style:

        XWRITE4(sc, door, XHCI_DOORBELL(0), 0);

which seems to get to:

ffff0000007c4b60 <generic_bs_w_4>     str   w3, [x1, x2]
ffff0000007c4b64 <generic_bs_w_4+0x4> ret

But I have not been able to track down there being the
apparently-architecturally-required DSB ST (or DSB)
after prior memory updates but before the write to the
doorbell/mailbox location (execution order for after
then before).

I'm using Barrier_Litmus_Tests_and_Cookbook_A08.pdf for
reference, section 8.1. But I'm not an aarch64 expert
so my interpretation of the material could be at issue.

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