Nico Huber uploaded patch set #3 to this change.
device/pci: Enable full 16-bit VGA port i/o decoding
So, the PCI to PCI bridge specification had a pitfall for us:
Originally, when decoding i/o ports for legacy VGA cycles, bridges
should only consider the 10 least significant bits of the port address.
This means all VGA registers were aliased every 1024 ports!
e.g. 0x3b0 was also decoded as 0x7b0, 0xbb0 etc.
However, it seems, we never reserved the aliased ports, resulting in
silent conflicts we preallocated resources. We neither use much
external VGA nor many i/o ports these days, so nobody noticed.
To avoid this mess, a bridge control bit (VGA16) was introduced in
2003 to enable decoding of 16-bit port addresses. As older systems
seem rather safe and well tested, and newer systems should support
this bit, we'll use it if possible and only warn if not.
With old (AGP era) hardware one will likely encounter a warning like
found VGA at PCI: 06:00.0
A bridge on the path doesn't support 16-bit VGA decoding!
This is not generally fatal, but makes unnoticed resource conflicts
Signed-off-by: Nico Huber <email@example.com>
4 files changed, 46 insertions(+), 1 deletion(-)
git pull ssh://review.coreboot.org:29418/coreboot refs/changes/16/35516/3
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