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

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Date:   Fri, 8 Dec 2023 16:44:00 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Mario Limonciello <mario.limonciello@....com>
Cc:     Sanath S <Sanath.S@....com>, bhelgaas@...gle.com,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Sanjay R Mehta <sanju.mehta@....com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Maciej W. Rozycki" <macro@...am.me.uk>
Subject: Re: [PATCH] PCI: Allocate maximum available buses to help extending
 the daisy chain

On Fri, Dec 08, 2023 at 04:29:42PM -0600, Mario Limonciello wrote:
> On 12/8/2023 16:24, Bjorn Helgaas wrote:
> > On Wed, Aug 16, 2023 at 10:49:23AM +0530, 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
> > 
> > Looks like a clear issue here because there's no other use for
> > buses 70-c1.  But what would happen if there were more hotplug-capable
> > downstream ports, e.g., assume one at 08.1 leading to [bus c2-c7]?
> > 
> > The 04.1 bridge has a lot of space, but 08.1 has very little.  With
> > this patch, would we distribute it more evenly across 04.1 and 08.1?
> > If not, I think we'll just have the same problem when somebody plugs
> > in a similar hierarchy at 08.1.
> > 
> > > In case of a thunderbolt capable bridge, reconfigure the buses allocated
> > > 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.
> > > 
> > > 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
> > 
> > This doesn't look like anything specific to Thunderbolt; it's just
> > that we don't do a good job of reassigning bus numbers in general,
> > right?  We shouldn't just punt and say "BIOS should have done
> > something" because not all machines *have* BIOS, and the OS can
> > reconfigure bus numbers as needed.  The patch certainly isn't
> > Thunderbolt-specific.
> 
> From the discussions Sanath and I have been in related to this issue
> the BIOS is pretty static with it's initialization under the
> presumption that the OS will rebalance things if necessary.
> ...

> For this particular issue it's being approached a different way.
> 
> Windows never rebalances things but doesn't suffer from this issue.
> That's because Windows actually does a "Downstream port reset" when
> it encounters a USB4 router.
> 
> Sanath posted a quirk that aligned this behavior when encountering
> an AMD USB4 router, but as part of the discussion I suggested that
> we do it for everyone.
> 
> https://lore.kernel.org/linux-usb/20231123065739.GC1074920@black.fi.intel.com/
> 
> So Sanath has a new patch that does this that is under testing right
> now and will be posted soon.

Hmm, ok.  I don't know what a "downstream port reset" does or how it
resolves the bus number allocation issue, but I'm happy if you have a
fix that doesn't need PCI core changes.

Bjorn