PCI Express devices which use legacy VGA compatibility should be placed on the Root Complex. This simplifies ioport access to VGA registers, which requires use of a special exception bit to work across PCI(e) bridges. It is also necessary for ioport access to VESA BIOS Extension (VBE) registers, which is not forwarded over PCI(e) bridges, even with the special exception bit for VGA register access.[1]
Update the PCI Express Guidelines to add these to the list of devices which can be placed directly on the Root Complex. Note that the only PCI Express display devices currently supported (bochs-display and virtio-gpu-pci) do not offer VGA compatibility. Legacy PCI devices (e.g. vga, qxl-vga, virtio-vga) are already documented as allowed on the Root Complex by the first item in the list. However, this item documents an additional consideration for placing devices which was not previously mentioned, and may be relevant for PCIe devices offering VGA compatibility in the future. [1]: https://mail.coreboot.org/hyperkitty/list/seab...@seabios.org/thread/XG2RN3HKVRDEDTLA2PRELLIENIIH7II7/#XVP3I2KQVZHSTDA4SNVKOITWGRGSDU3F Suggested-by: Laszlo Ersek <ler...@redhat.com> Signed-off-by: Kevin Locke <ke...@kevinlocke.name> --- Changes since v2: * Change Signed-off-by to Suggested-by for Laszlo Ersek, as suggested by Laszlo Ersek. Changes since v1: * Replace my overly-broad exception for devices requiring ioport access with a list item specifically for PCI Express devices offering VGA Compatibility provided by Laszlo Ersek. * Rewrite the commit message based on my improved understanding of the issue and the improved scope of the change. docs/pcie.txt | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/docs/pcie.txt b/docs/pcie.txt index 89e3502075..59b26817f9 100644 --- a/docs/pcie.txt +++ b/docs/pcie.txt @@ -48,13 +48,17 @@ Place only the following kinds of devices directly on the Root Complex: strangely when PCI Express devices are integrated with the Root Complex. - (2) PCI Express Root Ports (ioh3420), for starting exclusively PCI Express + (2) Assigned PCI Express GPUs that offer legacy VGA compatibility, and + that such compatibility is expected of (due to booting with SeaBIOS, + or due to UEFI driver bugs or native OS driver bugs). + + (3) PCI Express Root Ports (ioh3420), for starting exclusively PCI Express hierarchies. - (3) PCI Express to PCI Bridge (pcie-pci-bridge), for starting legacy PCI + (4) PCI Express to PCI Bridge (pcie-pci-bridge), for starting legacy PCI hierarchies. - (4) Extra Root Complexes (pxb-pcie), if multiple PCI Express Root Buses + (5) Extra Root Complexes (pxb-pcie), if multiple PCI Express Root Buses are needed. pcie.0 bus -- 2.35.1
