to remove a transaction from the async schedule. The previous method didn't
work well and led to the hardware writing to free'd buffers etc, as
it didn't always know that the transaction had been aborted.
Written after consultation with David Brownell who wrote the Linux
EHCI driver.
As part of this give the sqh structure a "previous" pointer.
MFC after: 1 week
system BIOS to disable legacy device emulation as per the "EHCI
Extended Capability: Pre-OS to OS Handoff Synchronisation" section
of the EHCI spec. BIOSes that implement legacy emulation using SMIs
are supposed to disable the emulation when this procedure is performed.
to be particularly correct or optimal, but it seems to be enough
to allow the attachment of USB2 hubs and USB2 devices connected via
USB2 hubs. None of the split transaction support is implemented in
our USB stack, so USB1 peripherals will definitely not work when
connected via USB2 hubs.
