Hi,
On 11/24/25 3:56 PM, zhangzhijie wrote:
> inb/outb speccial wire not support on other ARCH.
> Should detect whether arch platform support or not.
These platforms still have inb/outb definitions though that do
something. https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/1824
has a few cases where this goes wrong.
There are also a few Intel chipsets that also allow MMIO accesses to VGA
registers. Whether there is an intersection of chipsets that allow it,
and chipsets where that is useful is another question though.
> vga_get_uninterruptible(pdev, VGA_RSRC_LEGACY_IO);
This function really should check if inb/outb will address pdev after
the function returns, and return an error if not, then the accesses here
and in the power well code could be conditional on this, and it would be
generic.
For this, the vgaarb code would need to check that the bridges on the
way up have accepted the VGA forwarding bit (i.e. read back after
writing it), and there probably should be a mechanism for the bridge
between PCI(e) and the platform bus, so systems with multiple PCI(e)
domains can also be handled.
E.g. on POWER, there are global variables "isa_io_base" and
"pci_io_base", and it is assumed that these point to a place that
generates IO accesses from MMIO, but this assumption is not actually
verified, and I can pretty much guarantee that it's wrong if the PCIe
bus in question is not even attached to the first socket.
Just checking that upstream bridges accept the VGA bit while vgaarb
switches decoding over would solve all of my problems though, because
the root bridge doesn't, so that would be a quick way of inhibiting
these accesses. It might work for you too, just try setting the bit
manually with setpci and check if it sticks.
Simon
