Are back to back PCIe BAR allocations supported by Linux?

["Christian König" <christian.koenig@amd.com>]

Hello everyone,

we recently had a bug report of a system which works fine when a PCIe 
hotplug device is connected on boot, but fails to initialize if those 
device are disconnected and then reconnected again.

During investigation I've found that Linux isn't able to assign the BARs 
of the device correctly while reconnecting. The problem seems to be that 
the Linux PCI code doesn't seem to use back to back BAR allocations.

Now what's back to back BAR allocation? Let's assume you have two 
devices with a 256MiB BAR and a 2MiB BAR each behind a common upstream 
bridge.

The configuration Linux seems to use is the following:
Device A - 256MiB BAR
Device A -     2MiB BAR
Padding     254MiB
Device B - 256MiB BAR
Device B -     2MIB BAR

With padding this results in at least 770MiB address space requirement 
for the common upstream bridge, with alignment this is probably more 
like 1GiB.

The BIOS on the other hand seems to be capable of configuring the BARs 
like this:

Device A - 256MiB BAR
Device A -     2MiB BAR
Padding     252MiB
Device B -     2MIB BAR
Device B - 256MiB BAR

The result is that you only need 768MiB address space for the upstream 
bridge which then perfectly fits into what is assigned for hotplug here.

Is that already supported by the Linux PCIe code? If yes then how? I've 
tried to read a bit into the BAR allocation code, but it is kind of hard 
to understand.

Regards,
Christian.