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

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Date:   Fri, 18 Aug 2023 07:56:11 +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 Fri, Aug 18, 2023 at 09:01:50AM +0530, Sanath S wrote:
> 
> 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.

Right, but it would be good to get confirmation from the person who
reported the issue that this actually helps. There is nothing in the
bugzilla whether the patch worked or not.