On 8/31/26 18:58, Ilpo Järvinen wrote:
> On Mon, 31 Aug 2026, Christian König wrote:
...
>>> 2. Pack Oversized Windows (BAR Demotion):
>>> The PCI-to-PCI Bridge spec (r1.2, sec 3.2.5) permits a prefetchable BAR
>>> to be assigned from the non-prefetchable window. By clearing the PREFETCH
>>> flag on small control BARs during enumeration, they are placed below 4 GiB.
>>> The prefetchable window is then cleanly sized to the large BAR alone.
>>
>> Interesting hack, shouldn't really matter for AMD GPUs but that is
>> something more on the heuristic side.
>>
>> A perfectly valid trick which is done by BIOS implementations but what
>> the Linux PCI subsystem still hasn't learned are back to back
>> allocations, e.g. something like this:
>>
>> BAR0 32 GiB of GPU #1
>> BAR1 2 MiB of GPU #1
>>
>> BAR1 2 MiB of GPU #2
>> BAR0 32 GiB of GPU #2
>
> Assuming you're oversimplifying the layout above, this was added by the
> commit
> 9036bd0efcb6 ("PCI: Align head space better").
Oh nice! Yeah we had quite a number of bug reports because of that.
Usually hot unplugging some PCIe HW and then re-plugging it caused surprises
when the BIOS assigned the resources initially and then the Linux kernel on
re-plug.
I need to dig up those bug reports and add a reference to the commit to let
people retest.
Thanks for that pointer,
Christian.
