This bug prevents Alphas with older firmware from booting if there
is a card with PCI-PCI bridge that supports 32-bit IO.
This has happened on AS2100 with a quad-tulip card, for example:
- initially, the I/O window of 21152 bridge was 0x10000-0x10fff,
as set up by firmware;
- pci_setup_bridge() is going to change this, say, to 0xa000-0xafff:
first, it updates PCI_IO_BASE_UPPER16 and PCI_IO_LIMIT_UPPER16
registers, so that IO window temporarily is at 0x0000-0x0fff,
which effectively blocks up all legacy IO ports in the lower
4K range, such as serial, floppy, RTC an so on;
does debugging printk - machine dies here with recursive
machine checks as the serial console has gone.
Moving (or disabling) the debugging printk is not a solution -
there is possibility that timer interrupt (which might access RTC
ports) occurs between writes to lower and upper parts of the
base/limit registers.
The patch temporarily disables the IO window of the bridge by
setting PCI_IO_BASE_UPPER16 > PCI_IO_LIMIT_UPPER16 before doing
an update. It's safe, as we don't have any active IO behind
the bridge at this point. Also, it's a NOP for bridges with
16-bit-only IO.
Similar (but simpler, as we always clear upper 32 bits) fix
for 64-bit prefetchable MMIO range.
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