On 22 August 2017 at 23:19, Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > On Mon, Aug 21, 2017 at 11:53:01AM +0100, Lorenzo Pieralisi wrote: >> On Thu, Aug 17, 2017 at 09:30:28PM +1000, Daniel Axtens wrote: >> > A system without PCI legacy resources (e.g. ARM64) may find that no >> > default/boot VGA device has been marked, because the VGA arbiter >> > checks for legacy resource decoding before marking a card as default. >> >> I do not understand this paragraph, in particular: >> >> - "A system without PCI legacy resources (e.g. ARM64)". What does this >> mean ? I take this as "ARM64 does not support IO space"; if a PCI host >> bridge supports IO space, there is nothing from an architectural >> point of view that prevents an MMIO based IO space implementation to >> work on ARM64. It is PCI bridge specific, not arch specific. > > This reference to ARM64 is the same thing I stumbled over. Maybe it > could be written along the lines of: > > The VGA arbiter selects a default VGA device that is enabled and > reachable via the legacy VGA resources (mem 0xa0000-0xbffff, io > 0x3b0-0x3bb, io 0x3c0-0x3df, etc). > > If there is no such device, e.g., because there's no enabled VGA > device, the host bridge doesn't support access to those legacy > resources, or a PCI-PCI bridge doesn't have VGA Enable set, a > platform may select an arbitrary device by calling > vga_set_default_device(). > > Then I think this patch changes the previous behavior by allowing the > arbiter to select a device that is enabled and has a driver but may > not be reachable via the legacy VGA resources. > > I don't know what all the consequences of that would be, but it does > muddy the VGA arbiter waters a bit. AIUI, the main reason for the > arbiter is to allow multiple legacy VGA devices by manipulating bridge > VGA Enable bits so only one receives the legacy resources at a time. > > These devices that do not need the legacy resources don't need that > special treatment because they don't care about the bridge VGA Enable > bits and several can coexist in the system with no need for VGA > arbitration. > > I guess the powerpc fixup_vga() already selects a device that doesn't > need the VGA resources, so we already have that situation. > Perhaps it is unclear that the whole point of tinkering with the VGA arbiter is that X may refuse to use a GFX card if it is not tagged as the default VGA device by the arbiter. I have suggested before that fixing X may be more appropriate in this case, given that ownership of the legacy VGA resources may not correlate at all with being the primary display device on non-x86 architectures. _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
