Ping?
Am 01.04.21 um 14:03 schrieb Christian König:
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.
