Re: [PATCH] PCI: Allocate maximum available buses to help extending the daisy chain

[Sanath S]

On 8/17/2023 3:54 PM, Mika Westerberg wrote:
> On Wed, Aug 16, 2023 at 06:48:35PM +0530, Sanath S wrote:
> > Adding Mika.
> Thanks!
>
> > On 8/16/2023 10:49 AM, Sanath S wrote:
> > > In the case of Thunderbolt, it contains a PCIe switch and one or
> > > more hotplug-capable PCIe downstream ports where the daisy chain
> > > can be extended.
> > >
> > > Currently when a Thunderbolt Dock is plugged in during S5/Reboot,
> > > System BIOS allocates a very minimal number of buses for bridges and
> > > hot-plug capable PCIe downstream ports to enumerate the dock during
> > > boot. Because of this, we run out of bus space pretty quickly when
> > > more PCIe devices are attached to hotplug downstream ports in order
> > > to extend the chain.
> > >
> > > Before:
> > >              +-04.0
> > >              +-04.1-[63-c1]----00.0-[64-69]--+-00.0-[65]--
> > >              |                               +-01.0-[66]--
> > >              |                               +-02.0-[67]--
> > >              |                               +-03.0-[68]--
> > >              |                               \-04.0-[69]--
> > >              +-08.0
> This is something the BIOS should be doing but for some reason it is
> not on that particular system.
Yes, BIOS should be doing it. Idea here is if BIOS has not distributed 
it correctly, OS
can reallocate and distribute it correctly.
> > > In case of a thunderbolt capable bridge, reconfigure the buses allocated
> Thunderbolt
Will correct it.
> > > by BIOS to the maximum available buses. So that the hot-plug bridges gets
> > > maximum buses and chain can be extended to accommodate more PCIe devices.
> > > This fix is necessary for all the PCIe downstream ports where the daisy
> > > chain can be extended.
> This is necessary only when there is no proper BIOS allocation for the
> resources.
Yes, will send out a v2 with updated commit message.
> > > After:
> > >              +-04.0
> > >              +-04.1-[63-c1]----00.0-[64-c1]--+-00.0-[65]--
> > >              |                               +-01.0-[66-84]--
> > >              |                               +-02.0-[85-a3]--
> > >              |                               +-03.0-[a4-c0]--
> > >              |                               \-04.0-[c1]--
> > >              +-08.0
> > >
> > > Link: https://bugzilla.kernel.org/show_bug.cgi?id=216000
> Did you get confirmation that this actually solves the issue?
I've tested this on my setup, it is resolving the issue.
> > > Signed-off-by: Sanjay R Mehta <sanju.mehta@xxxxxxx>
> > > Signed-off-by: Sanath S <Sanath.S@xxxxxxx>
> > > ---
> > >    drivers/pci/probe.c | 9 +++++++++
> > >    1 file changed, 9 insertions(+)
> > >
> > > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > > index 8bac3ce02609..ab7e90ef2382 100644
> > > --- a/drivers/pci/probe.c
> > > +++ b/drivers/pci/probe.c
> > > @@ -1263,6 +1263,8 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
> > >    	bool fixed_buses;
> > >    	u8 fixed_sec, fixed_sub;
> > >    	int next_busnr;
> > > +	int start = bus->busn_res.start;
> > > +	int end = bus->busn_res.end;
> > >    	/*
> > >    	 * Make sure the bridge is powered on to be able to access config
> > > @@ -1292,6 +1294,13 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
> > >    		broken = 1;
> > >    	}
> > > +	/* Reconfigure, If maximum buses are not allocated */
> > > +	if (!pass && start != 0 && end != 0xff && subordinate != end) {
> > > +		pci_info(dev, "Bridge has subordinate 0x%x but max busn 0x%x, reconfiguring\n",
> > > +			 subordinate, end);
> > > +		broken = 1;
> > > +	}
> > > +
> > >    	/*
> > >    	 * Disable Master-Abort Mode during probing to avoid reporting of
> > >    	 * bus errors in some architectures.