Re: [RFT PATCH v3] PCI: move pci_read_bridge_bases to the generic PCI layer

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

On Thu, Jul 23, 2015 at 05:47:44PM +0100, Guenter Roeck wrote:
> On 07/23/2015 09:12 AM, Bjorn Helgaas wrote:
> > On Thu, Jul 09, 2015 at 11:59:16AM +0100, Lorenzo Pieralisi wrote:
> >> When a PCI bus is scanned, upon PCI bridge detection the kernel
> >> has to read the bridge registers to set-up its resources so that
> >> the PCI resource hierarchy can be validated properly.
> >>
> >> Most if not all architectures read PCI bridge registers in the
> >> pcibios_fixup_bus hook, that is called by the PCI generic layer
> >> whenever a PCI bus is scanned.
> >>
> >> Since pci_read_bridge_bases is an arch agnostic operation (and it
> >> is carried out on all architectures) it can be moved to the generic
> >> PCI layer in order to consolidate code and remove the respective
> >> calls from the architectures back-ends.
> >>
> >> The PCI_PROBE_ONLY flag is not checked before calling
> >> pci_read_bridge_buses in the generic layer since reading the bridge
> >> bases is not related to resources assignment; this implies that it
> >> can be carried out safely on PCI_PROBE_ONLY systems too and should
> >> not affect architectures (alpha, mips) that check the PCI_PROBE_ONLY
> >> flag before reading the bridge bases.
> >>
> >> Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> >
> > Applied to pci/resource for v4.3, thanks!
> >
> > The PCI_PROBE_ONLY text seems backward to me: previously alpha and mips
> > only called pci_read_bridge_bases() if PCI_PROBE_ONLY was set.  After this
> > patch, alpha and mips systems that do not set PCI_PROBE_ONLY will also call
> > pci_read_bridge_bases().
> >
> > I really don't know why alpha and mips were like that.  It seems backwards.
> > It seems like we'd want to know the bridge windows if we were assigning
> > things, but they only read them if they were *not* going to assign things.
> >
> > I'm a little uneasy that we might break some alpha or mips system, since
> > there must have been some reason this was done originally.  It'd be ideal
> > if somebody could test a non-PCI_PROBE_ONLY system.  But maybe they're all
> > obsolete.
> >
> 
> For alpha, PCI_PROBE_ONLY is set for two platforms, marvel and titan,
> out of ~20. mips sets the flag for 6 platforms out of >25.
> Unlikely that those are the only relevant ones.
> 
> I could try to run some qemu tests for both architectures, but I have
> no idea what to look out for. Ideas, anyone ?

As I replied to Bjorn, I would not expect pci_read_bridge_bases() to
be a disruptive operation on !PCI_PROBE_ONLY systems, I will check
again alpha and mips code but it is really hard to say, I will
inspect the code again to try to figure it out.

Thanks !
Lorenzo