spapr_pci: Advertise BAR reallocation capability

spapr_pci: Advertise BAR reallocation capability

The pseries guests do not normally allocate PCI resources and rely on
the system firmware doing so. Furthermore at least at some point in
the past the pseries guests won't even allowed to change BARs, probably
it is still the case for phyp. So since the initial commit we have [1]
which prevents resource reallocation.

This is not a problem until we want specific BAR alignments, for example,
PAGE_SIZE==64k to make sure we can still map MMIO BARs directly. For
the boot time devices we handle this in SLOF [2] but since QEMU's RTAS
does not allocate BARs, the guest does this instead and does not align
BARs even if Linux is given pci=resource_alignment=16@pci:0:0 as
PCI_PROBE_ONLY makes Linux ignore alignment requests.

ARM folks added a dial to control PCI_PROBE_ONLY via the device tree [3].
This makes use of the dial to advertise to the guest that we can handle
BAR reassignments. This limits the change to the latest pseries machine
to avoid old guests explosion.

We do not remove the flag from [1] as pseries guests are still supported
under phyp so having that removed may cause problems.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/platforms/pseries/setup.c?h=v5.1#n773
[2] https://git.qemu.org/?p=SLOF.git;a=blob;f=board-qemu/slof/pci-phb.fs;h=06729bcf77a0d4e900c527adcd9befe2a269f65d;hb=HEAD#l338
[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f81c11af
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20190719043734.108462-1-aik@ozlabs.ru>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>