Re: [PATCH v2 2/3] drivers: pci: host-generic: claim bus resources on PCI_PROBE_ONLY set-ups

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Mon, Apr 11, 2016 at 11:43:11PM -0500, Bjorn Helgaas wrote:
> Hi Lorenzo,
> 
> On Tue, Mar 01, 2016 at 02:44:08PM +0000, Lorenzo Pieralisi wrote:
> > The PCI host generic driver does not reassign bus resources on systems
> > that require the BARs set-up to be immutable (ie PCI_PROBE_ONLY) since
> > that would trigger system failures. Nonetheless, PCI bus resources
> > allocated to PCI bridge and devices must be claimed in order to be
> > validated and inserted in the kernel resource tree, but the current
> > driver omits the resources claiming and relies on arch specific kludges
> > to prevent probing failure (ie preventing resources enablement on
> > PCI_PROBE_ONLY systems).
> > 
> > This patch adds code to the PCI host generic driver that correctly
> > claims bus resources upon probe on systems that are required to
> > prevent reassignment after bus enumeration, so that the allocated
> > resources can be enabled successfully upon PCI device drivers probing,
> > without resorting to arch back-ends workarounds.
> > 
> > Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> > Acked-by: Will Deacon <will.deacon@arm.com>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: David Daney <david.daney@cavium.com>
> > Cc: Will Deacon <will.deacon@arm.com>
> > Cc: Bjorn Helgaas <bhelgaas@google.com>
> > ---
> >  drivers/pci/host/pci-host-generic.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/pci/host/pci-host-generic.c b/drivers/pci/host/pci-host-generic.c
> > index 1652bc7..e529825 100644
> > --- a/drivers/pci/host/pci-host-generic.c
> > +++ b/drivers/pci/host/pci-host-generic.c
> > @@ -252,7 +252,10 @@ static int gen_pci_probe(struct platform_device *pdev)
> >  
> >  	pci_fixup_irqs(pci_common_swizzle, of_irq_parse_and_map_pci);
> >  
> > -	if (!pci_has_flag(PCI_PROBE_ONLY)) {
> > +
> > +	if (pci_has_flag(PCI_PROBE_ONLY)) {
> > +		pci_bus_claim_resources(bus);
> > +	} else {
> >  		pci_bus_size_bridges(bus);
> >  		pci_bus_assign_resources(bus);
> >  
> 
> The next patch removes the arm and arm64 pcibios_enable_device()
> implementations, which implies that arm and arm64 only need the generic
> version, which simply calls pci_enable_resources().  That assumes r->parent
> is set.
> 
> After this patch, we'll call pci_bus_claim_resources() for the
> PCI_PROBE_ONLY case, and that sets r->parent for all the resources.
> 
> Where does r->parent get set in the non-PCI_PROBE_ONLY case?  Obviously
> that path *works*, because you're not changing anything there.  I'd just
> like to have a hint that makes this change more obvious.

On all ARM/ARM64 PCI controllers drivers I am aware of (apart from the
kvmtool PCI host controller which does require PCI_PROBE_ONLY, so we need
this patch), resources are always reassigned and the core code reassigning
them takes care of assigning their parent pointers too, to answer your
question.

As for this patch series, given that:

commit (in -next) 903589ca7165 ("ARM: 8554/1: kernel: pci: remove
pci=firmware command line parameter handling") removes the PCI_PROBE_ONLY
handling from the (ARM) command line, the PCI host generic becomes the
last ARM/ARM64 host controller that requires PCI_PROBE_ONLY to function
(depending on DT settings).

The idea behind adding pci_bus_claim_resources (patch 1) to core code
was that it could be reused by other arches too, I do not have evidence
though, I have to prove it, so I'd rather squash patch 1 into this one
and make the code claiming resources local to the PCI host generic,
I can't add a generic PCI core API just for one host controller
(IMHO we should add an API that allows us to claim bus resources and
realloc the ones for which claiming fail - which may mean releasing
bridges resources and realloc/resize them - code is in the kernel already
I have to write that API).

The code claiming resources on x86, IA64 and PowerPC looks extremely
similar but it has to be proven that a generic function has a chance
to work, so patch 1 is not really justified at present.

If you have no objections I will squash patch 1 into this one (moving
the respective code in PCI host generic driver), and I would not merge
this series till the commit above in -next gets in the kernel (which
makes sure that PCI_PROBE_ONLY can't be set on the command line, that's
fundamental to this series, at least on ARM, on ARM64 DT is the only way
PCI_PROBE_ONLY can be set and only on host controllers that check the
chosen node property - ie PCI host generic, that we are patching).

Thanks,
Lorenzo