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

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Message-ID: <20230817102430.GD3465@black.fi.intel.com>
Date:   Thu, 17 Aug 2023 13:24:30 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Sanath S <sanaths2@....com>
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, mario.limonciello@....com,
        Sanjay R Mehta <sanju.mehta@....com>
Subject: Re: [PATCH] PCI: Allocate maximum available buses to help extending
 the daisy chain

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.

> > In case of a thunderbolt capable bridge, reconfigure the buses allocated

Thunderbolt

> > 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.

> > 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?

> > Signed-off-by: Sanjay R Mehta <sanju.mehta@....com>
> > Signed-off-by: Sanath S <Sanath.S@....com>
> > ---
> >   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.