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
this:

    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
more likely.

Change-Id: Id7a07f069dd54331df79f605c6bcda37882a602d
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35516
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Michael Niewöhner
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

1 files changed, 5 insertions, 1 deletions

diff --git a/src/device/device.c b/src/device/device.c
index 44d1f95f75..333f1f0f1d 100644
--- a/src/device/device.c
+++ b/src/device/device.c
@@ -759,6 +759,10 @@ static void set_vga_bridge_bits(void)
 			continue;
 
 		printk(BIOS_DEBUG, "found VGA at %s\n", dev_path(dev));
+		if (dev->bus->no_vga16) {
+			printk(BIOS_WARNING,
+				"A bridge on the path doesn't support 16-bit VGA decoding!");
+		}
 
 		if (dev->on_mainboard) {
 			vga_onboard = dev;
@@ -797,7 +801,7 @@ static void set_vga_bridge_bits(void)
 	while (bus) {
 		printk(BIOS_DEBUG, "Setting PCI_BRIDGE_CTL_VGA for bridge %s\n",
 		       dev_path(bus->dev));
-		bus->bridge_ctrl |= PCI_BRIDGE_CTL_VGA;
+		bus->bridge_ctrl |= PCI_BRIDGE_CTL_VGA | PCI_BRIDGE_CTL_VGA16;
 		bus = (bus == bus->dev->bus) ? 0 : bus->dev->bus;
 	}
 }